usb: gadget: get rid of USB_GADGET_{DUAL,SUPER}SPEED

This commit removes USB_GADGET_DUALSPEED and USB_GADGET_SUPERSPEED
Kconfig options.  Since now kernel allows many UDC drivers to be
compiled, those options may turn to no longer be valid.  For
instance, if someone decides to build UDC that supports super
speed and UDC that supports high speed only, the latter will be
"assumed" to support super speed since USB_GADGET_SUPERSPEED will
be selected by the former.

The test of whether CONFIG_USB_GADGET_*SPEED was defined was just
an optimisation which removed otherwise dead code (ie. if UDC is
not dual speed, there is no need to handle cases that can happen
if speed is high).  This commit removes those checks.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>