libertas: fix invalid access

card->priv must not be accessed after lbs_remove_card() was called
as lbs_remove_card() frees card->priv via free_netdev().

For libertas_sdio this is a regression introduced by 23b149c1

.
The correct fix to the issue described there is simply to remove the
assignment. This flag is set at the appropriate time inside
lbs_remove_card anyway.

Reported-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Sven Neumann <s.neumann@raumfeld.com>
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>