spi: bcm-qspi: fix MSPI only access with bcm_qspi_exec_mem_op()

This fixes case where MSPI controller is used to access spi-nor
flash and BSPI block is not present.

Fixes: 5f195ee7

 ("spi: bcm-qspi: Implement the spi_mem interface")
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220328142442.7553-1-kdasu.kdev@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>