iwlwifi: mvm: fix A-MPDU reference assignment

The current code assigns the reference, and then goes to increment
it if the toggle bit has changed. That way, we get

Toggle  0  0  0  0  1  1  1  1
ID      1  1  1  1  1  2  2  2

Fix that by assigning the post-toggle ID to get

Toggle  0  0  0  0  1  1  1  1
ID      1  1  1  1  2  2  2  2

Reported-by: Danny Alexander <danny.alexander@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Fixes: fbe41127 ("iwlwifi: mvm: update mpdu metadata API")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>