libata: reorganize ata_ehi_hotplugged()

__ata_ehi_hotplugged() now has no users.  Regorganize
ata_ehi_hotplugged() such that a new function ata_ehi_schedule_probe()
deals with scheduling probing.  ata_ehi_hotplugged() calls it and
additionally marks hotplug specific flags.  ata_ehi_schedule_probe()
will be used laster.

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