spi: bcm-qspi: Always read and set BSPI_MAST_N_BOOT_CTRL

Always confirm the BSPI_MAST_N_BOOT_CTRL bit when enabling
or disabling BSPI transfers.

Fixes: 4e3b2d23

 ("spi: bcm-qspi: Add BSPI spi-nor flash controller driver")
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org