n_gsm: introduce enum gsm_dlci_state

n_gsm: introduce enum gsm_dlci_state

gsm_dlci->state is clearly an enumeration. So introduce one and use it
-- compiler now checks if valid values are assigned to the field.

Note that a compiler warns about unhandled cases in switch. Add default
cases with a pr_debug (which is not printed by default).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084949.28074-2-jslaby@suse.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>