i2c: make i2c_unregister_device() ERR_PTR safe

We are moving towards returning ERR_PTRs when i2c_new_*_device() calls
fail. Make sure its counterpart for unregistering handles ERR_PTRs as
well.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>