staging: dgap: remove useless variables for saving tty's major

The board_t has a tty_struct(serial_driver and print_driver)
that has a major number. When major number is compared in
dgap_tty_open(), just use brd->serial_driver{print_driver}->major.

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