spi/pl022: Explicitly truncate large bitmask

When building on 64 bit architectures the use of bitwise negation generates
constants larger than 32 bits which won't fit in u32s used to represent
32 bit register values on the device. Explicitly cast to let the compiler
know that the higher bits are not significant and can be discarded.

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