x86: fix is_io_mapping_possible() build warning on i386 allnoconfig

i386 allnoconfig:

 arch/x86/mm/iomap_32.c: In function 'is_io_mapping_possible':
 arch/x86/mm/iomap_32.c:27: warning: comparison is always false due to limited range of data type

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>