iwlwifi: pcie: fix A-MSDU on gen2 devices

The return status check of iwl_pcie_gen2_build_amsdu
was buggy. Fix it.

Fixes: 6ffe5de3 ("iwlwifi: pcie: add AMSDU to gen2")
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>