spi: microchip-core-qspi: fix setting spi bus clock rate

Before ORing the new clock rate with the control register value read
from the hardware, the existing clock rate needs to be masked off as
otherwise the existing value will interfere with the new one.

CC: stable@vger.kernel.org
Fixes: 8596124c

 ("spi: microchip-core-qspi: Add support for microchip fpga qspi controllers")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://lore.kernel.org/r/20240508-fox-unpiloted-b97e1535627b@spud
Signed-off-by: Mark Brown <broonie@kernel.org>