Revert "usb: musb: musb_cppi41: Handle ISOCH differently and not use the hrtimer."

This reverts commit 1af54b7a40ca9bbd549e626be01870caa3f0299d.

The commit tried to address cases in which isochronous transfers are 'not
reliable', most probably in the tests conducted, polling for the
MUSB_TXCSR_TXPKTRDY bit in MUSB_TXCSR is done too late.

Hence, it installs a work struct which basically busy-polls for the bit in a
rather agressive way by rescheduling the work if the FIFO is not empty. With
USB audio devices, tests have shown that it takes approximately 100
iterations of the asynchronous worker until the FIFO signals completion,
which leads to 100% CPU loads when streaming audio.

The issue the patch tried to address can be handled differently, which is
what the next patch does.

Signed-off-by: Daniel Mack <zonque@xxxxxxxxx>
Reported-by: Sebastian Reimers <sebastian.reimers@xxxxxxxxxxxxxx>
usb: musb: cppi41: fire hrtimer according to programmed channel length

The musb/cppi41 code installs a hrtimer to work around DMA completion
interrupts that have fired too early on AM335x hardware. This timer
is currently programmed to first fire 140 microseconds after the DMA
completion callback. According to the commit which introduced it
(a655f481d83, "usb: musb: musb_cppi41: handle pre-mature TX complete
interrupt"), that value is is considered a 'rule of thumb' that worked
well with the test case described in the commit log.

Test show, however, that for USB audio devices and much smaller packet
sizes, the timer has to fire earlier in order to correctly handle the audio
stream. The original test case had output transfer sizes of 1514 bytes, and
a delay of 140 microseconds. For audio devices with 24 bytes channel size, 3
microseconds seem to work well.

Hence, let's assume that the time it takes to clear the bit correlates with
the number of bytes transferred. The referenced commit log mentions such a
suspicion as well. Let the timer fire in cppi41_channel->total_len/10
microseconds to correctly handle both cases.

Also, shorten the interval in which the timer fires again in case of
a non-empty early_tx list.

With these changes in place, both FS and HS audio devices appear to work
well on AM335x hardware.

Signed-off-by: Daniel Mack <zonque@xxxxxxxxx>
Reported-by: Sebastian Reimers <sebastian.reimers@xxxxxxxxxxxxxx>
Merge branch 'fix/usb-isoc-3.15' into MA-E3xx/linux-3.15.y