OMAP: omap_device: replace _find_by_pdev() with to_omap_device()

The omap_device layer currently has two ways of getting an omap_device
pointer from a platform_device pointer.

Replace current usage of _find_by_pdev() with to_omap_device() since
to_omap_device() is more familiar to the existing to_platform_device()
used when getting a platform_device pointer from a struct device pointer.

Cc: Felipe Balbi <balbi@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>
Reviewed-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>