MIPS: CMP: Fix physical core number calculation logic

The CPUNum Field in EBase register is 10bit wide, so after 1 bit right
shift, the mask value should be 0x1ff.

Signed-off-by: jerin jacob <jerinjacobk@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4420/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>