Call `yield` in `ModemClass::waitForResponse` loop

[Duality4Y]

consider adding a yield statement here:

https://github.com/arduino-libraries/MKRNB/blob/c0ee4ed2fd3cad28a2072e0e27e7fd6b80affa89/src/Modem.cpp#L308

This is a tight loop and I don't think real-time timing is really all that necessary here.