iwlwifi: pcie: fix support for transmitting SKBs with fraglist

iwlwifi: pcie: fix support for transmitting SKBs with fraglist

When the implementation of SKBs with fraglist was sent upstream, a
merge-damage occurred and half the patch was not applied.

This causes problems in high-throughput situations with AX200 devices,
including low throughput and FW crashes.

Introduce the part that was missing from the original patch.

Fixes: 0044f1716c4d ("iwlwifi: pcie: support transmitting SKBs with fraglist")
Cc: stable@vger.kernel.org # 4.20+
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[ This patch was created by me, but the original author of this code
  is Johannes, so his s-o-b is here and he's marked as the author of
  the patch. ]
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>