spi: atmel: Add missing annotation for atmel_spi_next_xfer_dma_submit()

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>

drivers/spi/spi-atmel.c

@@ -706,6 +706,7 @@ static void atmel_spi_next_xfer_pio(struct spi_master *master,
 static int atmel_spi_next_xfer_dma_submit(struct spi_master *master,
 				struct spi_transfer *xfer,
 				u32 *plen)
+	__must_hold(&as->lock)
 {
 	struct atmel_spi	*as = spi_master_get_devdata(master);
 	struct dma_chan		*rxchan = master->dma_rx;