minstrel_ht: remove unnecessary NULL check in minstrel_ht_update_caps

If sta is NULL, we will have problems long before we get here...

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Cc: Felix Fietkau <nbd@openwrt.org>