staging: dgap: Rename driver

Renames driver file dgap_driver.c and dgap_driver.h to
dgap.c and dgap.h because we are now single source and
include file and better fits kernel naming conventions.

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