drivers: modem: cellular: correct user DLCI channels

The supported modems don't have the same number or assignment
of DLCI channels. For example, the NRF91_SLM only has DLCI
channel 3 assigned to GNSS tunneling, where the ublox sara r5
has DLCI 3 assigned to an additional AT channel, and DLCI 4
assigned to GNSS tunneling.

This commit updates the creation and assignment of DLCI
channels and pipelinks to match the capabilties of each modem.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>