usb: gadget: add a inline function gether_bitrate()

In function ecm_bitrate(), it is not support report bit rate for super
speed plus mode, but it can use same bit rate value defined in ncm and
rndis.

Add a common inline function gether_bitrate() which report different for
all possible speeds, it can be used by ecm, ncm and rndis, also remove
old function from them.

Signed-off-by: Linyu Yuan <quic_linyyuan@quicinc.com>
Link: https://lore.kernel.org/r/20230803091053.9714-3-quic_linyyuan@quicinc.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>