mfd: Make twl6030_irq_set_wake static

twl6030_irq_set_wake is not used anywhere else in the kernel
except as irq_chip.irq_set_wake. No reason for it to be exported.

Also fixes build warning:
drivers/mfd/twl6030-irq.c:230:5: warning: symbol 'twl6030_irq_set_wake' was not declared. Should it be static?

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>