Cellular: fix athandler to use correct timeout in case of multiple urc's
Cellular: fix athandler to use correct timeout in case of multiple urc's

Start time was not updated when multiple urc's were found and so timeout
was set to zero. Now that start time is updated, timeout is correct.

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