powerpc/mm: Parenthesise IS_ENABLED() in if condition

Currently IS_ENABLED() produces an expression surrounded by parentheses,
which allows this code to compile, generating eg:

    else if (1 || 0)
        hpte_init_native();

However a change to the macro in the kbuild tree will break this in
future by removing the parentheses.

Fixes: 7353644f ("powerpc/mm: Fix build break when PPC_NATIVE=n")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>