clocksource/drivers/mips-gic-timer: Use correct shift count to extract data

__gic_clocksource_init() extracts the GIC_CONFIG_COUNTBITS field from
read_gic_config() by right shifting the register value. The shift count is
determined by the most significant bit (__fls) of the bitmask which is
wrong as it shifts out the complete bitfield.

Use the least significant bit (__ffs) instead to shift the bitfield down to
bit 0.

Fixes: e07127a0 ("clocksource: mips-gic-timer: Use new GIC accessor functions")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: daniel.lezcano@linaro.org
Cc: paul.burton@imgtec.com
Link: https://lkml.kernel.org/r/20180228095610.50341-1-nbd@nbd.name