usb: gadget: dummy_hcd: add 'is_high_speed' parameter

This patch adds a new module parameter to dummy_hcd
called is_high_speed.

When set to false the connected device will be forced
to operate in full-speed mode. By default, this parameter
is set to 'true'.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>