staging: dgap: Fix various previously missed checkpatch errors

This patch fixes various small checkpatch errors
I missed in patches 01-10.

Remove check for kfree of brd->channels as kfree is safe.
Also there is no need to set it to NULL after freeing it.

Remove a useless cast for a kzalloc return value.

Remove double negative test in a white space fix line.

Signed-off-by: Mark Hounschell <markh@compro.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>