AT_CellularDevice.cpp - unused variable err warning
AT_CellularDevice.cpp - unused variable err warning

We get this compiler warning;

Compile [  7.9%]: AT_CellularDevice.cpp
[Warning] AT_CellularDevice.cpp@206,18: variable 'err' set but not used [-Wunused-but-set-variable]

Due to the fact, that the code that would actually use this variable
is behind trace flags. Based on review feedback from Antti Kauppila and
Kimmo Väisänen, adding same flagging also for that.
- Reason being - the operation needs to be within the locks.

mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os