mfd: Fix generic irq chip ack function name for jz4740-adc

In commit 659fb32d

("genirq: replace irq_gc_ack() with {set,clr}_bit
variants"), irq_gc_ack was renamed to irq_gc_ack_set_bit. The jz4740-adc
driver still uses the old name which results in a compile error.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>