[PATCH] ahci: convert to new probing mechanism and add hotplug support

Convert to new probing mechanism and add hotplug support by enabling
PORT_IRQ_PHYRDY, marking ehi for hotplug and scheduling EH on
CONNECT/PHYRDY interrupts.

Unfortunately, ahci cannot reliably wait for the first D2H FIS after
hotplug.  It sometimes succeeds but times out more often than not, so
ATA_FLAG_SKIP_D2H_BSY is used.

This patch also fixes ahci_hardreset() such that D2H Register FIS RX
area is cleared before issuing COMRESET.  Without this,
ata_busy_sleep() after COMRESET might prematually finish if the
previous TF contains DRDY && !BSY.

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