mfd: Fix mismatch in twl4030 mutex lock-unlock

A mutex is locked on entry into twl4030_madc_conversion().
Immediate return on some error conditions leaves the
mutex locked.

This patch ensures that mutex is always unlocked before
leaving the function.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>