staging: gdm7240: a TTY rewrite according to the latest TTY APIs

Fixed mis-use of mutex for gdm_table. gdm_table is refered to only
inside tty_install and port destrcut, and usb callbacks use internal
reference which was saved during urb submission

Signed-off-by: Won Kang <wonkang@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>