Convert sata_sil24 to new EH.

* When port is frozen, IRQ for the port is masked.

* sil24_softreset() doesn't need to mangle with IRQ mask anymore.
  libata ensures that the port is frozen during reset.

* Only turn on interrupts which are handled by interrupt handler and
  EH.  As we don't handle SDB notify yet, turn it off. DEV_XCHG and
  UNK_FIS are handled by EH and thus turned on.

* sil24_softreset() usually fails to recover the port after DEV_XCHG.
  ATA_PORT_HARDRESET is used as recovery action for DEV_XCHG.

* sil24 may be invoked without any active command.  e.g. DEV_XCHG irq
  occuring while no qc in progress still triggers EH and will reset
  the port and revalidate attached device.
Signed-off-by: Tejun Heo <htejun@gmail.com>