mac80211: reduce stack usage in sta_ht_capa_read()

The maximal size of the "ht_capa" file is 430 bytes.  In most cases,
it's much shorter.  Use a 512 byte long buffer.  1024 bytes is too much
and causes a warning with CONFIG_FRAME_WARN=1024.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>