staging: dgap: use schedule_timeout_interruptible() instead of dgap_ms_sleep()

Using schedule_timeout_interruptible() is exactly same as
setting a status of current process and calling  schedule_timeout().

Removes dgap_ms_sleep(), because this function is used
only when closing tty channel on dgap_tty_close().
And also removes ch_close_delay that is always set to 250
on dgap_tty_init().

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