staging: dgnc: clean up allocation of ->channels[i]

Check if kzalloc fails in dgnc_tty_init() and if it does then free all
previously allocated ->channels[i] and set them to NULL. This makes the code
less error/bug prone because instead of needing programmers attention to add
checks everywhere we do that in one place. Also, remove a bogus comment and
check in the same loop because ->channels[i] isn't allocated anywhere else.
Finally, remove a unnecessary check if ->channels[i] is NULL in the next loop
because it can't be.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>