usb: gadget:audio: Replace deprecated macro S_IRUGO

Use octal digits as suggested by checkpatch instead of the deprecated
macro.

Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20220121155308.48794-2-pavel.hofman@ivitera.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>