Move SGPIO ISRs to separate .c/.h files.
Move usb_bulk_buffer* to separate .c/.h files.
Convert rf_path_set_direction from if/else to switch/case.
Add RF_PATH_DIRECTION_OFF enum and code.
Update RFFC5071 enable state when switching RF path direction. TODO: There's some stuff here that needs refactoring -- duplicate code is showing up.
Remove commented RFFC5071 code in set_transceiver_mode.
Set RF path direction to "off" when transceiver_mode is OFF.
Set transceiver mode to OFF whenever USB configuration changes.