MIPS: Remove redundant instructions from arch_spin_{,try}lock.

We were doing:
   SRL  $r,$?,16
   ANDI $r,$r,0xffff

The logical right shift by 16 leaves the upper 16 bits clear, so the
subsequent masking out of those bits is redundant, and can safely be
removed.

Signed-off-by: David Daney <david.daney@cavium.com>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>