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

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>