Add config_ep_by_speed() to configure the endpoint
according to the gadget speed.

Using this function will spare the FDs from handling
the endpoint chosen descriptor.
Signed-off-by: NTatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>