greybus: audio: Use underscore in file name

For consistency with most other files in the Greybus repository,
change 'audio-codec.c' to use an underscore instead of a hyphen
in its name.

Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>