iwlwifi: mvm: delay enabling smart FIFO until after beacon RX

If we have no beacon data before association, delay smart FIFO
enablement until after we have this data.

Not doing so can cause association failures in extremely silent
environments (usually only a shielded box/room) as beacon RX is
not sent to the host immediately, and then the association time
event ends without the host receiving any beacon even though it
was on the air - it's just stuck on the FIFO.

Cc: <stable@vger.kernel.org> [3.14]
Fixes: 1f3b0ff8 ("iwlwifi: mvm: Add Smart FIFO support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>