spi: bcm2835: bcm2835_spi_handle_err(): fix NULL pointer deref for non DMA transfers

spi: bcm2835: bcm2835_spi_handle_err(): fix NULL pointer deref for non DMA transfers

In case a IRQ based transfer times out the bcm2835_spi_handle_err()
function is called. Since commit c3ad4e82d4ce ("spi: bcm2835: Drop
dma_pending flag") the TX and RX DMA transfers are unconditionally
canceled, leading to NULL pointer derefs if ctlr->dma_tx or
ctlr->dma_rx are not set.

Fix the NULL pointer deref by checking that ctlr->dma_tx and
ctlr->dma_rx are valid pointers before accessing them.

Fixes: c3ad4e82d4ce ("spi: bcm2835: Drop dma_pending flag")
Cc: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20220719072234.2782764-1-mkl@pengutronix.de
Signed-off-by: Mark Brown <broonie@kernel.org>