Patch from Ben Dooks

Check that the device we are looking at is really
a platform device before trying to cast it to one
to find out the platform bus number.

Thanks to RMK for pointing this out.
Signed-off-by: NBen Dooks <ben-linux@fluff.org>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>