usb: dwc3: ep0: avoid empty-body warning

Building with W=1, we get a warning about harmless empty statements:

drivers/usb/dwc3/ep0.c: In function 'dwc3_ep0_handle_intf':
drivers/usb/dwc3/ep0.c:491:4: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]

Instead of adding empty braces which would introduce checkpatch.pl
warnings, we're just removing the code which doesn't do anything and
making sure we return 0 so USBCV tool is happy.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>