Cellular: Fix AT URC handler not to dismiss incoming chars

[AriParkkila]

Description

Fixes AT URC handler to not miss URC in a case when a part of that was received just before URC timeout expired, for example:

I------------I-------------I
NO CARRIER

This fix depends on PR #7030.

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2182
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7033/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 1970
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7033/1970

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1807
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7033/

[AnttiKauppila]

@0xc0170 This is ready to go in