staging: gdm724x: Clarify naming of packet_type<->tty index symbols

staging: gdm724x: Clarify naming of packet_type<->tty index symbols

This driver was using "packet_type" for packet types and for
the mapping of TTY indices to packet types.

Fix the confusion by renaming the symbols.

Fixes sparse warning:

drivers/staging/gdm724x/gdm_mux.c:146:24: warning: symbol 'packet_type' shadows an earlier one

Signed-off-by: Misha Gusarov <dottedmag@dottedmag.net>
Link: https://lore.kernel.org/r/20201024202221.60726-1-dottedmag@dottedmag.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>