wifi: radiotap: separate vendor TLV into header/content

To be able to use a general function later for any kind of
TLV, separate the vendor TLV header/content in the structs.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230305124407.8ac5195bb3e6.I19ad99c1ad3108453aede64bddf6ef1a7c4a0b74@changeid


[separate from the original combined patch]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>