sh: mach-x3proto: Improve ILSEL debugging.

At the moment ILSEL blows up with a BUG when aliased sets are handed in,
but as the enable call is able to hand back errors we opt for that path
instead. None of the ILSEL peripherals are vital to the board's
operation, so trapping a BUG is a bit excessive.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>