iwlwifi: mvm: add L-SIG length to radiotap

We may have the L-SIG length depending on the phy_data info type;
add it to radiotap when we do.

Move getting the phy_data out one layer up and the info type into
it so we can use this data more generically. We need to call the
iwl_mvm_rx_he() function for other reasons as well, so can't just
combine all of that into something like iwl_mvm_parse_phy_data().

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