powerpc/mmu: Don't duplicate radix_enabled()

powerpc/mmu: Don't duplicate radix_enabled()

mmu_has_feature(MMU_FTR_TYPE_RADIX) can be evaluated regardless of
CONFIG_PPC_RADIX_MMU.

When CONFIG_PPC_RADIX_MMU is not set, mmu_has_feature(MMU_FTR_TYPE_RADIX)
will evaluate to 'false' at build time because MMU_FTR_TYPE_RADIX
wont be included in MMU_FTRS_POSSIBLE.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/62743846cbd493e5d9a02e197c2672a1d30df149.1620366342.git.christophe.leroy@csgroup.eu