Sparse reports a warning at atmel_spi_next_xfer_dma_submit()

warning: context imbalance in atmel_spi_next_xfer_dma_submit()
	- unexpected unlock

The root cause is the missing annotation
	at atmel_spi_next_xfer_dma_submit()

Add the missing __must_hold(&as->lock) annotation
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Link: https://lore.kernel.org/r/20200429225723.31258-3-jbi.octave@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>