staging: pi433: Fix validation of rf69_get_modulation value

Checking of modulation in rf69_set_modulation_shaping is done by
if-else and since else part covers OOK and UNDEF values it possible to
set modulation shaping for undefined modulation type.
To fix this validation should be done by switch clause and in case of
undefined modulation error returned.

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