staging: dgnc: make find_board_by_major static

Fixes the following sparse warning:
drivers/staging/dgnc/dgnc_tty.c:933:19: warning: symbol 'find_board_by_major' was not declared. Should it be static?

Signed-off-by: Benoit Hiller <benoit.hiller@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>