spi: pxa2xx: Fix too early chipselect deassert

There is a chance that chipselect is deasserted too early while the last
clock cycle is still running. Protocol analyzers will see this as a failed
last byte. This is more likely to occur with slow bitrates, for instance
at 25 kbps.

Reason for this is when using SPI mode 0 that both SPI host controller and
SPI slave will drive the data lines at the falling edge of clock signal
and sample at the rising edge. Receive FIFO gets the last bit now at the
rising edge and code sees transfer to be finished either by the interrupt
in PIO mode or by the DMA completion in DMA mode.

The SSP Time Out register SSTO should take care of delaying the
completion but it does not seems to have effect at least on Intel
Skylake and Broxton even when using long enough values. Depending on
timing code may get into point where chipselect is deasserted while the
last clock cycle is still running at its second half cycle.

Fix this by adding a wait loop in giveback() that waits until SSP becomes
idle before deasserting the chipselect.

Reported-by: Weifeng Voon <weifeng.voon@intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>