irqchip/mips-gic: Fix shifts to extract register fields

The MIPS GIC driver is incorrectly using __fls to shift registers,
intending to shift to the least significant bit of a value based upon
its mask but instead shifting off all but the value's top bit. It should
actually be using __ffs to shift to the first, not last, bit of the
value.

Apparently the system I used when testing commit 3680746a
("irqchip: mips-gic: Convert remaining shared reg access to new
accessors") and commit b2b2e584 ("irqchip: mips-gic: Clean up mti,
reserved-cpu-vectors handling") managed to work correctly despite this
issue, but not all systems do...

Fixes: 3680746a ("irqchip: mips-gic: Convert remaining shared reg access to new accessors")
Fixes: b2b2e584 ("irqchip: mips-gic: Clean up mti, reserved-cpu-vectors handling")
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: https://lkml.kernel.org/r/20170922062440.23701-2-paul.burton@imgtec.com