ARM: sun4i: spi: Allow transfers larger than FIFO size

SPI transfers were limited to one FIFO depth, which is 64 bytes.
This was an artificial limitation, however, as the hardware can handle
much larger bursts. To accommodate this, we enable the interrupt when
the Rx FIFO is 3/4 full, and drain the FIFO within the interrupt
handler. The 3/4 ratio was chosen arbitrarily, with the intention to
reduce the potential number of interrupts.

Since the SUN4I_CTL_TP bit is set, the hardware will pause
transmission whenever the FIFO is full, so there is no risk of losing
data if we can't service the interrupt in time.

This patch only handles long Rx transfers, and continues to return
-EINVAL on long Tx transfers. Tx transfers will be handled in a
subsequent patch.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>