Add a cast in case the caller passes in a different type, as it would
if mtspr/mtmsr were functions.

Previously, if a 64-bit type was passed in on 32-bit, GCC would bind the
constraint to a pair of registers, and would substitute the first register
in the pair in the asm code.  This corresponds to the upper half of the
64-bit register, which is generally not the desired behavior.
Signed-off-by: NScott Wood <scottwood@freescale.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>