Using INPUT_PULLUP as an argument for digitalWrite is risky at best as a
method to enable the pullup on a pin (valid parameters are 0 (LOW) or 1
(HIGH). INPUT_PULLUP has a value of 2). Setting pullups should be done
thorugh pinMode.
See here:

https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/Arduino.h

https://github.com/arduino/Arduino/blob/master/hardware/arduino/avr/cores/arduino/wiring_digital.c

Replaced the low level SPI driver for a more generic RX5808 one.
This should allow for more flexible control of receivers and later
addition of new features, including individual control of receivers,
setting arbitrary frequencies, and generally improving code structure.

…y' into Knifa/tidy-up

As requested, replaced tabs with spaces and lowercase function names

Formatting changes, as requested