sh: Fix up the SH-4A mutex fastpath semantics.

This fixes up the __mutex_fastpath_xxx() routines to match the semantics
noted in the comment. Previously these were looping rather than doing a
single-pass, which is counter-intuitive, as the slow path takes care of
the looping for us in the event of contention.

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