staging: pi433: make local functions static

Following functions:
* rf69_get_modulation
* rf69_read_reg
* rf69_write_reg
are used locally only and should be declared as static

Signed-off-by: Marcin Ciupak <marcin.s.ciupak@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>