spi: atmel: Reduce spin lock usage

The current implementation of the driver holds a spin lock for the
duration of the transfer, releasing it only to enable interrupts for
short periods of time.  As this would prevent any interrupt from
happening, this could cause system performance issues every time a SPI
message is sent.  Since the spi core now handles message syncronization
we can reduce the amount of time the spin-lock is held to the regions
where both the calling thread and the interrupt might interract.

Signed-off-by: Dan Sneddon <dan.sneddon@microchip.com>
Link: https://lore.kernel.org/r/20210602160816.4890-2-dan.sneddon@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>