irqchip/mips-gic: Use bitfield helpers

Use the FIELD_GET() helper, instead of open-coding the same operation.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/all/74f9d126961a90d3e311b92a54870eaac5b3ae57.1637593297.git.geert+renesas@glider.be