usb: gadget: use working speed to calcaulate network bitrate and qlen

Take ecm_bitrate() as example, it will be called after gadget device
link speed negotiation, consider code
if (gadget_is_superspeed(g) && g->speed == USB_SPEED_SUPER),
if a gadget device link speed is USB_SPEED_SUPER,
gadget_is_superspeed(g) must be true, or not it is a wrong
configuration of gadget max support speed.

Remove gadget_is_superspeed(g) checking should be safe, and remove other
similar operation in ncm, rndis, u_ether.

Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/20230803091053.9714-2-quic_linyyuan@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>