iwlwifi: remove CMD_WANT_SKB flag if send_cmd_sync failure

In function iwl_send_cmd_sync(), if the flag CMD_WANT_SKB is set but
we are not provided with a valid SKB (cmd->meta.u.skb == NULL), we need
to remove the CMD_WANT_SKB flag from the TX cmd queue. Otherwise in case
the cmd comes in later, it will possibly set an invalid address. Thus
it causes an invalid memory access.

This fixed the bug http://bugzilla.kernel.org/show_bug.cgi?id=11326

.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>