iwlwifi: tx_sync only on PAN context

Ted reported that he couldn't connect to some APs
and bisected it to the tx_sync implementation.
Disable it for the BSS context to fix this issue.

Reported-by: Ted Ts'o <tytso@mit.edu>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>