regulator: max8952: removed unused mutex.

This patch removes a mutex that is never used in the driver.

Reported-by: Axel Lin
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>