hwmon: (pmbus/core) mutex_lock write in pmbus_set_samples

update_lock is a mutex intended to protect write operations. It was not
taken, however, when _pmbus_write_word_data is called from
pmbus_set_samples() function which may cause problems especially when
some PMBUS_VIRT_* operation is implemented as a read-modify-write cycle.

This patch makes sure the lock is held during the operation.

Fixes: 49c4455d ("hwmon: (pmbus) Introduce PMBUS_VIRT_*_SAMPLES registers")
Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
[groeck: Declared and initialized missing 'data' variable]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>