x86: Fix the address space annotations of iomap_atomic_prot_pfn()

This patch fixes the sparse warnings when the return pointer of
iomap_atomic_prot_pfn() is used as an argument of iowrite32()
and friends.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
LKML-Reference: <1283633804-11749-1-git-send-email-currojerez@riseup.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>