drivers: modem_cellular: changes due to net_link_addr struct changes

Recent changes to net_link_addr structure to not use pointers
have made it necessary to update the modem_cellular driver
to ensure compatibility with the new structure definition.
Otherwise, an assertion failure occurs in subsys/net/l2/ppp/ipv6cp.c:40.
This commit updates that link address is set to NET_LINK_ADDR_MAX_LENGTH
least significant bytes of IMEI instead of IMEI total length.

Signed-off-by: Jani Hirsimäki <jani.hirsimaki@nordicsemi.no>