powerpc/mm: Switch from __FUNCTION__ to __func__

__FUNCTION__ exists only for backwards compatibility reasons with old
gcc versions. Replace it with __func__.

Signed-off-by: Dwaipayan Ray <dwaipayanray1@gmail.com>
[chleroy: Fixed parenthesis alignment]
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210711084536.95394-1-dwaipayanray1@gmail.com