[PATCH] libata: don't schedule EH on wcache on/off if old EH

Do not schedule EH for revalidation on wcache on/off if old EH.  Old
EH cannot handle it and will result in WARN_ON()'s and oops.

This closes bug #7412.

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