mISDN: Fix HDLC DSP transmit

Fix HDLC DSP transmit (DL_DATA frames were bounced back upwards instead of
being sent down as PH_DATA frames)

Thanks to Andreas Eversberg for the fix!

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Peter Schlaile <root@asterisk.schlaile.de>
Signed-off-by: Karsten Keil <kkeil@suse.de>