usb: mtu3: introduce a funtion to check maximum speed

The max_speed got from DTS may be not supported by the
hardware, so need check it, and assign a proper default
value. And make it clear by introducing a funtion.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1595834101-13094-5-git-send-email-chunfeng.yun@mediatek.com


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