mISDN: Fix TEI and SAPI handling

Added SAPI value to use SAPIs different than 0.

Now fixed TEIs work in NT mode. This allows PTP endpoint to be connected
to PTMP ports together with other PTMP endpoints.

New enhanced version, thanks to Sam Ravnborg <sam@ravnborg.org> for the
hints.

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>