spi: atmel: Drop unused variable

The DMA cap mask is no longer used since:
commit 7758e390 ("spi: atmel: remove compat for non DT board when requesting dma chan")
Drop it now.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20210218132840.131898-1-tudor.ambarus@microchip.com


Signed-off-by: Mark Brown <broonie@kernel.org>