staging: greybus: remove extern prototypes

Functions are implicitly declared with "extern". The compiler doesn't
need it.  extern removed from drivers/staging/greybus/audio_codec.h
file.

Signed-off-by: Payal Kshirsagar <payal.s.kshirsagar.98@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>