sh: Allocate PMB entry slot earlier

Simplify set_pmb_entry() by removing the possibility of not finding a
free slot in the PMB. Instead we now allocate a slot in pmb_alloc() so
that if there are no free slots we fail at allocation time, rather than
in set_pmb_entry().

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>