Staging: dgap: Remove obsolete driver

Remove support for dgap driver since there is no way to get
the firmware files required by the dgap driver into the linux-
firmware tree. The dgap driver is rendered useless without this
firmware and hence this product is considered obsolete by DIGI.

Signed-off-by: Shraddha Barke <shraddha.6596@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>