[PATCH] libata: implement ATA_FLAG_IN_EH port flag

ATA_FLAG_IN_EH flag is set on entry to EH and cleared on completion.
This patch just sets and clears the flag.  Following patches will
build normal qc execution / EH synchronization aroung this flag.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>