staging: dgap: fix kernel oops on port open

This patch addresses the follow error message followed
by a kernel oops:

dgap: driver does not set tty->port. This will crash the kernel later. Fix the driver

It also renames the main function this patch addresses because
its name is misleading.

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