iio: dac: ad3530r: #undef field_prep() before local definition

iio: dac: ad3530r: #undef field_prep() before local definition

Prepare for the advent of a globally available common field_prep() macro
by undefining the symbol before defining a local variant.  This prevents
redefinition warnings from the C preprocessor when introducing the common
macro later.

Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>