bmc150_magn_remove() always returns zero. Make it return no value which
makes it easier to see in the callers that there is no error to handle.

Also the return value of i2c driver remove callbacks is ignored anyway.
This prepares making i2c remove callbacks return void, too.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220514133250.307955-1-u.kleine-koenig@pengutronix.deSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>