[PATCH] libata: implement ap->sata_spd_limit and helpers

ap->sata_spd_limit contrains SATA PHY speed of the port.  It is
initialized to the configured value prior to probing thus preserving
BIOS configured value.  hardreset is responsible for applying SPD
limit and sata_std_hardreset() is updated to do that.  SATA SPD limit
will be used to enhance failure handling during probing and later by
EH.

This patch also normalizes some comments around affected code.

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