usb: mtu3: use boolean return value

Prefer to use boolean value due to gpd_ring_empty() return true or false.
See "16) Function return values and names" in coding-style.rst

Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20230417025203.18097-2-chunfeng.yun@mediatek.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>