staging: dgnc: move functions unrelated with dgnc_found_board()

The functions related with tty device initialization are needed
to be moved from dgnc_found_board() to dgnc_init_one().

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