sata_via: Implement hotplug for VT6421

sata_via: Implement hotplug for VT6421

Enable IRQ on hotplug and add an interrupt handler to handle it.

This allows hotplug to work:
ata5: exception Emask 0x10 SAct 0x0 SErr 0x70000 action 0xe frozen
ata5: SError: { PHYRdyChg PHYInt CommWake }
ata5: hard resetting link
ata5: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
ata5.00: LPM support broken, forcing max_power
ata5.00: ATA-7: WDC WD800JD-75MSA3, 10.01E04, max UDMA/133
ata5.00: 156250000 sectors, multi 0: LBA48 NCQ (depth 0/32)
ata5.00: LPM support broken, forcing max_power
ata5.00: configured for UDMA/133
ata5: EH complete
scsi 4:0:0:0: Direct-Access     ATA      WDC WD800JD-75MS 1E04 PQ: 0 ANSI: 5
sd 4:0:0:0: [sdb] 156250000 512-byte logical blocks: (80.0 GB/74.5 GiB)
sd 4:0:0:0: [sdb] Write Protect is off
sd 4:0:0:0: [sdb] Mode Sense: 00 3a 00 00
sd 4:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
sd 4:0:0:0: Attached scsi generic sg1 type 0
sd 4:0:0:0: [sdb] Attached SCSI disk

And also hot unplug:
ata5: exception Emask 0x10 SAct 0x0 SErr 0x1b0000 action 0xe frozen
ata5: SError: { PHYRdyChg PHYInt 10B8B Dispar }
ata5: hard resetting link
ata5: SATA link down (SStatus 0 SControl 310)
ata5: hard resetting link
ata5: SATA link down (SStatus 0 SControl 310)
ata5: hard resetting link
ata5: SATA link down (SStatus 0 SControl 310)
ata5.00: disabled
ata5: EH complete
ata5.00: detaching (SCSI 4:0:0:0)
sd 4:0:0:0: [sdb] Synchronizing SCSI cache
sd 4:0:0:0: [sdb] Synchronize Cache(10) failed: Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
sd 4:0:0:0: [sdb] Stopping disk
sd 4:0:0:0: [sdb] Start/Stop Unit failed: Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Tejun Heo <tj@kernel.org>