libertas: be more careful about command responses matching cur_cmd

Especially in the light of OLPC trac #5461, in which the firmware starts
sending us seemingly random command responses which bear little relation
to the command we sent it.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>