MIPS: Netlogic: Fix sign extension in PIC write

This does not cause a problem yet, but we do not want to write 1
to reserved bits.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5424/


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