hwmon: (pmbus_core) Use guard() for mutex protection

hwmon: (pmbus_core) Use guard() for mutex protection

Simplify the code by using guard() and scoped_guard() instead of
mutex_lock()/mutex_unlock() sequences.

This patch changes semantics for debugfs accesses. Previously, those
used mutex_lock_interruptible() and not mutex_lock(). This change is
intentional and should have little if any impact since locks should not
be held for a significant amount of time and debugfs accesses are less
critical than sysfs accesses (which never used interruptable locks).

Reviewed-by: Sanman Pradhan <psanman@juniper.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>