mfd: core: Fix device reference leak in mfd_clone_cell

Make sure to drop the reference taken by bus_find_device_by_name()
before returning from mfd_clone_cell().

Fixes: a9bbba99 ("mfd: add platform_device sharing support for mfd")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>