iio:core: Introduce unlocked version of iio_map_array_unregister()

Introduce an unlocked version of iio_map_array_unregister(). This function
can help to unwind in case of error while the iio_map_list_lock mutex is
held.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/1606571059-13974-1-git-send-email-LinoSanfilippo@gmx.de
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>