irqchip: mips-cpu: Replace magic 0x100 with IE_SW0

irqchip: mips-cpu: Replace magic 0x100 with IE_SW0

Replace use of the magic number 0x100 (ie. bit 8) with the more
explanatory IE_SW0 (ie. interrupt enable for software interrupt 0) or
C_SW0 (ie. cause bit for software interrupt 0) as appropriate.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15834/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>