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

In the Linux kernel, the following vulnerability has been resolved:

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 1513ceee70f2 ("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.