usb: dwc3: qcom: suppress unused-variable warning

usb: dwc3: qcom: suppress unused-variable warning

The dwc3_qcom_read_usb2_speed() helper is now only called when the
controller is acting as host, but the compiler will warn that the hcd
variable is unused in gadget-only W=1 builds.

Fixes: c06795f114a6 ("usb: dwc3: qcom: fix gadget-only builds")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20220822100550.3039-1-johan+linaro@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>