[PATCH] libata-hp-prep: implement ap->hw_sata_spd_limit

Add ap->hw_sata_spd_limit and initialize it once during the boot
initialization (or driver load initialization).  ap->sata_spd_limit is
reset to ap->hw_sata_spd_limit on hotplug.  This prevents spd limits
introduced by earlier devices from affecting new devices.

Signed-off-by: Tejun Heo <htejun@gmail.com>