Fix compile errors

RF24Network.h:357:74: sorry, unimplemented: non-static data member
initializers
RF24Network.h:357:74: error: ISO C++ forbids in-class initialization of
non-const static member âmax_frame_payload_sizeâ

RF24Network.cpp:297:9: warning: unknown conversion type character 0x20
in format [-Wformat]
RF24Network.cpp:297:9: warning: unknown conversion type character 0x20
in format [-Wformat]
RF24Network.cpp:297:9: warning: too many arguments for format
[-Wformat-extra-args]