watchdog: mena21_wdt: Drop unnecessary mutex lock

There is already a mutex in the watchdog core which serializes
calls to the various API functions.
So the mutex lock "drv->lock" is unnecessary and can be dropped.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Johannes Thumshirn <jth@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>