iwlwifi: remove late null-check and duplicate bug_on

These pieces of code appear to be useless. The BUG_ON is also performed in
iwl_send_cmd_async.

Serious karma would be needed to enter iwl_ativate_qos with a null priv.
I had a deja vu when patching this, but for the life of me I couldn't track
down a similar patch.

Signed-off-by: Ian Schram <ischram@telenet.be>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>