[PATCH] libata: implement ata_dev_enabled and disabled()

This patch renames ata_dev_present() to ata_dev_enabled() and adds
ata_dev_disabled().  This is to discern the state where a device is
present but disabled from not-present state.  This disctinction is
necessary when configuring transfer mode because device selection
timing must not be violated even if a device fails to configure.

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