When ata_set_mode() fails on a device, make ata_set_mode() return
error code and pointer to the device instead of disabling it directly.
This gives more control to higher level driving logic.

This patch does not change the end result (configured transfer mode)
although it may make libata repeat mode configuration to the peer of a
failing device.  Later ata_bus_probe() rewrite will make full use of
this change.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>