powerpc/32: replace MTMSRD() by mtmsr

On PPC32, MTMSRD() is simply defined as mtmsr.

Replace MTMSRD(reg) by mtmsr reg in files dedicated to PPC32,
this makes the code less obscure.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/22469e78230edea3dbd0c79a555d73124f6c6d93.1576916812.git.christophe.leroy@c-s.fr