iwlwifi: pcie: support transmitting SKBs with fraglist

We want to be able to build A-MSDUs in higher layers, e.g. by
xmit_more, so support transmitting SKBs with fraglist to use
it for such.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>