libertas: fix potential NULL-pointer dereference

The code wants to check if there's a channel and it is not disabled,
but it used to check if channel is not NULL and accessed the channel
struct if this check failed.

Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>