counter: 104-quad-8: Add lock guards - differential encoder

Add lock protection from race conditions to 104-quad-8 counter driver
for differential encoder status code changes. Mutex lock calls used for
protection.

Signed-off-by: Syed Nayyar Waris <syednwaris@gmail.com>
Fixes: 954ab5cc ("counter: 104-quad-8: Support Differential Encoder Cable Status")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>