spi: bcm-qspi: Make BSPI default mode

The spi-nor controller defaults to BSPI mode, hence switch back
to its default mode after MSPI operations (write or erase)
are completed.

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Kamal Dasu <kdasu.kdev@gmail.com>
Link: https://lore.kernel.org/r/1567139325-7912-1-git-send-email-rayagonda.kokatanur@broadcom.com


Signed-off-by: Mark Brown <broonie@kernel.org>