wifi: libertas: fix a couple of sparse warnings

 - endian swapping is required in one place, use the
   already swapped 'bsssize' local
 - lbs_disablemesh need not be exported and can be static

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220904212910.76c34b2ae7a0.Ieb97c72b6d26f9d695cc4ab10fa7af5c3509612b@changeid