spi: bcm-qspi: check for valid cs before applying chip select

Apply only valid chip select value. This change fixes case where chip
select is set to initial value of '-1' during probe and PM supend and
subsequent resume can try to use the value with undefined behaviour.
Also in case where gpio based chip select, the check in
bcm_qspi_chip_select() shall prevent undefined behaviour on resume.

Fixes: fa236a7ef240 ("spi: bcm-qspi: Add Broadcom MSPI driver")
Signed-off-by: Kamal Dasu <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-bcm-qspi.c

···
 	u32 rd = 0;
 	u32 wr = 0;
 
-	if (qspi->base[CHIP_SELECT]) {
+	if (cs >= 0 && qspi->base[CHIP_SELECT]) {
 		rd = bcm_qspi_read(qspi, CHIP_SELECT, 0);
 		wr = (rd & ~0xff) | (1 << cs);
 		if (rd == wr)