staging: dgnc: drop unused neo_uart_struct

Now that the "neo" portion of the driver is gone (it was never being
used), drop the unused neo_uart_struct variable as well.

Cc: Lidza Louina <lidza.louina@gmail.com>
Cc: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>