n_gsm: drop unneeded gsm_dlci->fifo field

gsm_dlci->fifo always points to gsm_dlci->_fifo. So drop the pointer and
rename _fifo to fifo. And update all the users (add & to them).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084949.28074-1-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>