MIPS: Fix harmlessly missing else statement.

The actual bug is a missing else statement - but really this should be
expressed using a switch() statement.

Found by Al Viro who writes "the funny thing is, it *does* work only
because r2 is syscall number and syscall number around 512 => return
value being ENOSYS and not one of ERESTART...  so we really can't hit
the first if and emerge from it with ERESTART_RESTARTBLOCK.  still
wrong to write it that way..."

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