ion: use PAGE_KERNEL instead of pgprot_kernel

The ion code uses PAGE_KERNEL (which is more generic than
pgprot_kernel) everywhere except in one place (so fix it).

Change-Id: I38fa1c682a3e04e9c8cb3cd25678094862fe8c9c
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>