ath5k: fix some comment typos

Fix comment about dma sizes, brackets were missing. Replace 'insure' with
'ensure'.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>