[MIPS] Fix O32 personality(2) call with 0xffffffff argument.

A sign extension bug did result in sys_personality being invoked with a
0xffffffffffffffffUL argument, so querying the current personality didn't
work.

Signed-off-by: Thiemo Seufer <ths@networkno.de>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>