powerpc/mm: Fix typo in IS_ENABLED()

IS_ENABLED() matches names exactly, so the missing "CONFIG_" prefix
means this code would never be built.

Also fixes a missing newline in pr_warn().

Fixes: 970d54f9 ("powerpc/book3s64/hash: Disable 16M linear mapping size if not aligned")
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/202006050717.A2F9809E@keescook