iio:potentiostat:lmp91000: change dev_err message

Change dev_err message on line 215 in order to inform that
tia-gain-ohm is not defined and an external resistor is not
specified.

Signed-off-by: Anderson Reis <andersonreisrosa@gmail.com>
Signed-off-by: Lucas Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>