ALSA: usb-audio: #undef field_{get,prep}() before local definition

Prepare for the advent of globally available common field_get() and
field_prep() macros by undefining the symbols before defining local
variants.  This prevents redefinition warnings from the C preprocessor
when introducing the common macros later.

Suggested-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

Author: geertu

sound/usb/mixer_quirks.c

@@ -3312,7 +3312,9 @@ static int snd_bbfpro_controls_create(struct usb_mixer_interface *mixer)
 #define RME_DIGIFACE_INVERT BIT(31)
 
 /* Nonconst helpers */
+#undef field_get
 #define field_get(_mask, _reg) (((_reg) & (_mask)) >> (ffs(_mask) - 1))
+#undef field_prep
 #define field_prep(_mask, _val) (((_val) << (ffs(_mask) - 1)) & (_mask))
 
 static int snd_rme_digiface_write_reg(struct snd_kcontrol *kcontrol, int item, u16 mask, u16 val)