iio: magnetometer: bmc150_magn: use automated cleanup for mutex

iio: magnetometer: bmc150_magn: use automated cleanup for mutex

Use guard() and scoped_guard() to replace manual mutex lock/unlock
calls. This simplifies error handling and ensures RAII-style cleanup.

guard() is used in read_raw, write_raw, trig_reen, and
trigger_set_state. Case blocks using guard() in read_raw and write_raw
are wrapped in braces at the case label level to ensure clear scope for
the cleanup guards.

A bmc150_magn_set_power_mode_locked() helper is added to deduplicate
the lock-call-unlock pattern used by remove, runtime_suspend, suspend,
and resume.

The trigger_handler function is left unchanged as mixing guard() with
goto error paths can be fragile.

Signed-off-by: Neel Bullywon <neelb2403@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>