x86: remove unnecessary save_mr() sanity check

Impact: cleanup

The save_mr() function already checks that start_pfn is less than
end_pfn so we can remove the unnecessary check which reduces the
diff between the 32-bit and the 64-bit versions of init_memory_mapping().

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1236257708-27269-6-git-send-email-penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>