iwlagn: simplify iwlagn_tx_skb

We can simplify length calculation in iwlagn_tx_skb, that function
is enough complex, without fuzz it more than necessary.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>