lora: sx12xx_common: transition to k_poll_signal

Transition the receive synchronisation method from a single driver
semaphore to a function specific k_poll_signal. This is required to
allow the modem to be released without introducing race conditions on
the signalling mechanism.

Without this change, the RX can either be signalled before the modem is
released, unblocking the calling thread before the modem is put back to
sleep, or after the modem is released, in which case a second thread
could start using the semaphore before the original thread is signalled.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>