Reuse schedRx2() for RX1 as well
Reuse schedRx2() for RX1 as well

This reduces some code duplication, and prepares for an upcoming change
that makes the scheduling a bit more complex.

To reflect the more generic usage of this function, it is renamed to
schedRx12.

mcci-catena/arduino-lmic,mcci-catena/arduino-lmic,mcci-catena/arduino-lmic