[rilmodem] check that received numeric value is valid

Existing numeric value with invalid content seems to be able to
cause dbus crash. This fix prevents the sending if the value does
not match with ril specification.

Signed-off-by: Jussi Kangas <jussi.kangas@oss.tieto.com>
Revert "[rilmodem] Make LTE configuration forcing working both ways"

This reverts commit 7d8c819d6b2d11acab365a0b9a1eb8f1c586adb7.