sata_vsc: refactor vsc_sata_interrupt and hook up error handling

sata_vsc: refactor vsc_sata_interrupt and hook up error handling

Separate sata_vsc interrupt handling into a normal (per-port) path and an
error path with the addition of vsc_port_intr and vsc_error_intr
respectively.  The error path handles interrupt based
hotplug events which requires the definition of vsc_freeze and vsc_thaw.

Note: vsc_port_intr has a workaround for unexpected interrupts that occur
during polled commands.  This fixes a regression between 2.6.19 and 2.6.20.

Changes in take2:
* removed definition of invalid fis bit
* let standard ata-error-handling handle the serror register
* clear all unhandled interrupts
* revert changes to vsc_intr_mask_update (vsc_thaw enables all interrupts)
* use unlikely() for the pci-abort and not-our-interrupt cases in vsc_sata_interrupt

Changes in take3:
* Unify the "add" + "hook-up" patches into this single patch

[htejun@gmail.com: clean up comments and suggestions]
Cc: Jeremy Higdon <jeremy@sgi.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>