usb: gadget: dummy_hcd: use the shared_hcd infrastructure

This patch is a preparation for adding SuperSpeed
support to dummy hcd.

It takes the master side fields out of the struct
dummy to a separate structure. The init process
was also modified to resemble the way it is
done by xHCI.
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>