spi: spi-ti-qspi: Fix error handling

'dma_request_chan_by_mask()' can not return NULL.
Try to keep the logic in 'no_dma:' by resetting 'qspi->rx_chan' in case
of error.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>