staging: dgap: unwind on error in dgap_tty_init()

If the kzalloc() fails for channels, it need to handle
that error. It should free channels which were already
allocated.

And also removes the call to dgap_tty_uninit() in
dgap_firmware_load().

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