Use a common begin() method that can be called from derived classes
to contain the shared initialization code.
Signed-off-by: Frieder Schrempf <frieder@fris.de>
According to the documentation and experiments on other boards using
NRF52 + SX1262 this reduces the power consumption significantly.
Signed-off-by: Frieder Schrempf <frieder@fris.de>