Revert "OMAP: omap_device: add omap_device_is_valid()"

This reverts commit 0007122a

.

The dereference method of checking for a valid omap_device when
wrapping a platform_device is rather unsafe and dangerous.

Instead, a better way of checking for a valid omap-device is
to use a common parent device for all omap_devices, then a check
can simply be made using the device parent.  The only user of this
API was the initial version of the runtime PM core for OMAP.  This
has now been switched to check device parent, so there are no more
users of this API.

Acked-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>