Staging: iio: meter: fixed typo

This patch fixes an obvious typo, which will cause erroneously returning the Peak
Voltage instead of the Peak Current.

Signed-off-by: Leonard Pollak <leonardp@tr-host.de>
Cc: <Stable@vger.kernel.org>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>