powerpc/32s: Change mfsrin() into a static inline function

mfsrin() is a macro.

Change in into an inline function to avoid conflicts in KVM
and make it more evolutive.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/72c7b9879e2e2e6f5c27dadda6486386c2b50f23.1612612022.git.christophe.leroy@csgroup.eu