counter: 104-quad-8: Add lock guards - filter clock prescaler

Add lock protection from race conditions to the 104-quad-8 counter
driver for filter clock prescaler code changes. Mutex calls used for
protection.

Signed-off-by: Syed Nayyar Waris <syednwaris@gmail.com>
Fixes: de65d055 ("counter: 104-quad-8: Support Filter Clock Prescaler")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>