usb: gadget: qdss: Fix usage on non-DWC3 controllers

The QDSS function needs to call into controller-specific
routines to adjust parameters such as FIFO size and endpoint
configuration, but currently this only needs to be when using
the DWC3 controller. Previously this was handled by calling
stub functions if CONFIG_USB_DWC3_MSM is not defined. However,
this breaks targets that have it defined and compiled but do
not use the DWC3 controller as the primary gadget, as it
results in calling the routines of an uninstantiated driver.

Fix this by guarding the DWC3 calls by first checking
if (gadget_is_dwc3(gadget)).

Change-Id: I8fe045410bbb5dbe11cf4c14d06e6fba9351dd1b
Signed-off-by: Jack Pham <jackp@codeaurora.org>