drivers: sensor: lis3mdl: return -ENOTSUP on unsupported channels

sensor_channel_get() API should return -ENOTSUP when requested channel
is not supported. This behavior allows to use `sensor get DEVNAME` shell
command easily, as all unsupported channels are filtered out.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>