staging: dgap: remove bogus null test in dgap_tty_init()

- The channels array were set to NULL in dgap_found_board().
- Removes redundant null check for channels array in for loop,
if one of the channel cannot be allocated, dgap_tty_init() just returns
an error.

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