mfd: Remove unneeded io_mutex from struct twl6040

Current code has been converted to use regmap APIs, the io_mutex is not needed.
Thus remove the io_mutex.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>