wifi: iwlwifi: mvm: simplify by using SKB MAC header pointer

Instead of calculating the offset to the 802.11 header based on radiotap
bits and length, shorten the code path by always setting the MAC header
in the skb and using skb_mac_header().

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Link: https://lore.kernel.org/r/20230127002430.3ec5493934a4.I1d41a2af28588b5899fcd2402f8c4bd8cc29a12e@changeid
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>