ath9k_htc: Fix array overflow

Use ENDPOINT_MAX instead of HST_ENDPOINT_MAX.
This fixes a stack corruption issue.

This is based on a patch sent by Dan Carpenter <error27@gmail.com>.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>