USB: serial: digi_acceleport: rename tty flag variable

Add a "tty_" prefix to the tty "flag" variable to avoid any future
mixups with the recently added irq-mask "flags" one.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>