usb: gadget: renesas_usbhs: add usbhs_bus_get_speed()

current mod_gadget had got usb speed on
usbhsg_irq_dev_state() which is status change interrupt callback function.
And the usb speed data was included in its parameter.

But this style works for mod_gadget,
but doesn't work for mod_host which
isn't interrupted when device status was changed.

This patch add usbhs_bus_get_speed() to solve this issue.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>