powerpc/mm: Fix hugetlb page initialization

This patch fixes a regression by using correct kernel config variable
for HUGETLB_PAGE_SIZE_VARIABLE.

Without this huge pages are disabled during kernel boot.
[0.309496] hugetlbfs: disabling because there are no supported hugepage sizes

Fixes: c5710cd2 ("powerpc/mm: cleanup HPAGE_SHIFT setup")
Reported-by: Sachin Sant <sachinp@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>