staging: dwc2: Don't touch the dma_mask when dma is disabled

There was some code that cleared the dma_mask when dma was disabled in
the driver. Given that clearing the mask doesn't actually tell the usb
core we're not using dma, and a previous commit explicitely sets the
hcd->self.uses_dma value, it seems these values are unneeded and can
only potentially cause problems (when reloading a module, for example).

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <Paul.Zimmerman@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>