staging: dgnc: introduce find_board_by_major()

It was used to get a board structure with dgnc_BoardsByMajor array.
But this driver already has the array for managing initialized board
as dgap_board[]. It can be used for searching the board structure
by major number.

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