MIPS: Netlogic: Use PRID_IMP_MASK macro

Use PRID_IMP_MASK macro instead of 0xff00 to extract the processor
type.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6868/


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