uqmi: add timeout option value

This value will be used for now during following situations:
* Ask the sim with the uqmi --get-pin-status command.
* Wait for network registration with the uqmi --get-serving-system command.

This two commands wait forever in a while loop. Add a timeout to stop
waiting and so inform netifd.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>