staging: dgap: introducing find_board_by_major()

use find_board_by_major function instead of getting a brd from static
arrary. Becasue tty's major number doesn't start zero and we can
find a brd from dgap_board[].

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>