devres: WARN() and return, don't crash on device_del() of uninitialized device

devres: WARN() and return, don't crash on device_del() of uninitialized device

I just debugged an obscure crash caused by a device_del() of a all NULL'd
out struct device (in usb-serial) and found that a patch like this one would
have saved me time (in addition to improved chances of a bug report from
users hitting similar driver bugs).

[[email protected]: cleanup]
Signed-off-by: Benjamin Herrenschmidt <[email protected]>
Cc: Kay Sievers <[email protected]>
Cc: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>