iwlwifi: pcie: gen2: build A-MSDU only for GSO

If the incoming frame should be an A-MSDU, it may already be one,
for example in the case of NAN multicast being encapsulated in an
A-MSDU. Thus, use the GSO algorithm to build A-MSDU only if the
skb actually contains GSO data.

Fixes: 6ffe5de3 ("iwlwifi: pcie: add AMSDU to gen2")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>