MIPS: Watch: Avoid duplication of bits in mips_read_watch_registers

Currently the bits to be masked when watchhi is read is defined inline
for each register. To avoid this, define the bits once and mask each
register with that value.

Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Acked-by: David Daney <david.daney@cavium.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18158/


Signed-off-by: James Hogan <jhogan@kernel.org>