greybus: add write retry support for i2c

It is expected that i2c writes may fail, and in that case the driver
simply retries some number of times before actually treating it as a
failure.  Define a GB_OP_RETRY status, which is interpreted by the
i2c driver as an indication a retry is in order.  We just translate
that into an EAGAIN error passed back to the i2c core.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>
greybus: update gbuf status for completion handlers

Currently, if a USB urb completes with an error, that error status
is not transferred back to the gbuf that it's associated with.  For
inbound data there's not a lot we can do about an error, but for
outbound data, this means there is no notification to the submitter
that something went wrong.

For outbound data copy the urb status directly back to the gbuf as
its status.  Follow USB's lead and set the status to -EINPROGRESS
while a gbuf is "in flight."  Assign a gbuf an initial status value
of -EBADR to help identify use of never-set status values.

When an inbound urb fails (SVC or CPort), currently the urb is just
leaked, more or less (i.e., we lose an urb posted to receive
incoming data).  Change that so such an error is reported, but
then re-submitted.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>