spi: spi_qsd: Fix unmasked write to IO control register

As part of the shared QUP core code path the driver makes an unmasked
register write to the SPI IO control register overwriting all the other
important control bits. Fix this write to be a masked write.

Change-Id: Ief6b196b169ca1b651ef3074ac1c6b4412a8450f
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>