spi: qup: Fix cs-num DT property parsing

num-cs is 32 bit property, don't read just upper 16 bits.

Fixes: 4a8573ab

 (spi: qup: Remove chip select function)
Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org