iio: adc: cc10001: Fix regulator_get_voltage() return value check

regulator_get_voltage() returns a non-negative value in case of success,
and a negative error in case of error. Let's fix this.

Fixes: 1664f6a5 ("iio: adc: Cosmic Circuits 10001 ADC driver")
Signed-off-by: Naidu Tellapati <naidu.tellapati@imgtec.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>