Make sure digit events are not reported as "ERROR"

dahdievent_to_analogevent used a simple switch statement to convert DAHDI
event numbers to "ANALOG_*" event numbers. However "digit" events
(DAHDI_EVENT_PULSEDIGIT, DAHDI_EVENT_DTMFDOWN, DAHDI_EVENT_DTMFUP)
are accompannied by the digit in the low word of the event number.

This fix makes dahdievent_to_analogevent() return the event number as-is
for such an event.

This is also required to fix #15924 (in addition to r222108).  


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@222237 65c4cc65-6c06-0410-ace0-fbb531ad65f3