OMAP: omap_device: ensure hwmod tracks attached omap_device pointer

The omap_hwmod struct has a field to track the omap_device that is
attached to it, but it was not being assigned.  Fix by assigning omap_device
pointer when omap_device is built.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
[paul@pwsan.com: use an array index rather than pointer arithmetic]
Signed-off-by: Paul Walmsley <paul@pwsan.com>