staging: iio: meter: Remove reduntant __func__ from debug print

dev_dbg includes the function name & line number by default when dynamic
debugging is enabled. Hence__func__ is reduntant here and removed.

Signed-off-by: HariPrasath Elango <hariprasath.elango@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>