tty: n_gsm, remove duplicates of parameters

dp, f, and i are only duplicates of gsmld_receive_buf's parameters. Use
the parameters directly (cp, fp, and count) and delete these local
variables.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-41-jslaby@suse.cz


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