usb: dwc2: Fix channel disable flow

Channel disabling/halting should performed for enabled only channels
to avoid warnings "Unable to clear enable on channel N" which seen
if host works in Slave mode.

Signed-off-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>