ASoC: cs35l56: Use PCI SSID to select specific

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

The PCI device registers contain a subsystem ID (SSID), that is
separate from the silicon ID. The PCI specification defines it thus:

"They provide a mechanism for board vendors to distiguish their
 boards from one another even thought the boards may have the same
 PCI controller on them."

This allows the driver for the silicon part to apply board-speficic
settings based on this SSID.

The CS35L56 driver uses this to select the correct firmware file for
the board. The actual ID is part of the PCI register set of the
host audio interface so this set of patches includes extracting the
SSID from the Intel audio controller and passing it to the machine
driver and then to ASoC components. Other PCI audio controllers
will have the same SSID registers, so can use the same mechanism to
pass the SSID.