hwmon: Simplify the locking model of two drivers

Many hardware monitoring drivers use two different mutexes, one to
protect their per-device data structure, and one to protect the
access to the device registers. These mutexes are essentially
redundant, as the drivers are transfering values between the device
registers and the data cache, so they almost always end up holding
both mutexes at the same time. Using a single mutex will make the
code more simple and faster.

I am changing only two of the affected drivers here, the authors
of the other affected drivers are welcome to submit similar patches
if they want.

Signed-off-by: Jean Delvare <khali@linux-fr.org>