staging: pi433: Fix missing 'undefined' value in enum modulation

It is possible that rf69_get_modulation() function will return
'undefined' value and this value is missing in enum modulation. Fix this
by adding appropriate entry in enum modulation.

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