libata: clear ehi description after initial host report

ehi description field is used to carry LLD specific controller
description.  Sometimes, it's used without clearing before and LLD
description gets printed with exception information one more time.
Clear after printing.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>