m68k: Allow ioremapping top of memory

The test in __ioremap to reject memory ranges crossing the 0 boundary
rejects also memory ranges ending at the end of the memory.  Fix that.

Signed-off-by: Philippe De Muyter <phdm@macqel.be>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>