mfd: Drop twl4030-irq's mask_work

... we can do the synchronization with the
hardware when calling bus_sync_unlock as
we're supposed to. While at that, also make
variable names uniform on all functions.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>