libertas: clear current command on card removal

If certain commands were in-flight when the card was pulled or the
driver rmmod-ed, cleanup would block on the work queue stopping, but the
work queue was in turn blocked on the current command being canceled,
which didn't happen.  Fix that.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Cc: stable <stable@kernel.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>