powerpc/mpic: Use bitmap_zalloc() when applicable

'mpic->protected' is a bitmap. So use 'bitmap_zalloc()' to simplify
code and improve the semantic, instead of hand writing it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/aa145f674e08044c98f13f1a985faa9cc29c3708.1639777976.git.christophe.jaillet@wanadoo.fr