wifi: libertas: only add RSN/WPA IE in lbs_add_wpa_tlv

The existing code only converts the first IE to a TLV, but it returns a
value that takes the length of all IEs into account. When there is more
than one IE (which happens with modern wpa_supplicant versions for
example), the returned length is too long and extra junk TLVs get sent
to the firmware, resulting in an association failure.

Fix this by finding the first RSN or WPA IE and only adding that. This
has the extra benefit of working properly if the RSN/WPA IE isn't the
first one in the IE buffer.

While we're at it, clean up the code to use the available structs like
the other lbs_add_* functions instead of directly manipulating the TLV
buffer.

Signed-off-by: Doug Brown <doug@schmorgal.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230123053132.30710-3-doug@schmorgal.com