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>

Author: geertu

drivers/iio/dac/ad3530r.c

@@ -54,6 +54,7 @@
 #define AD3531R_MAX_CHANNELS			4
 
 /* Non-constant mask variant of FIELD_PREP() */
+#undef field_prep
 #define field_prep(_mask, _val)	(((_val) << (ffs(_mask) - 1)) & (_mask))
 
 enum ad3530r_mode {