staging: dgap: Merge dgap_tty.c into dgap_driver.c

There is a lot of cleanup work to do on these digi drivers and merging as
much as is possible will make it easier. I also notice that many merged
drivers are single source and header.

Signed-off-by: Mark Hounschell <markh@compro.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>