MIPS: Trap exception handling fixes

2a0b24f5 broke Trap exception handling in
the standard MIPS mode.  Additionally the microMIPS-mode trap code mask is
wrong, as it's a 4-bit field.  Here's a fix.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5309/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>