mac80211: minstrel_ht: move short preamble check out of get_rate

Test short preamble support in minstrel_ht_update_caps instead of
looking at the per-packet flag. Makes the code more efficient.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>