usb: gadget: composite: switch over to ERR_CAST()

This patch fixes the following Coccinelle warning:

drivers/usb/gadget/composite.c:1142:9-16: WARNING: \
	ERR_CAST can be used with uc

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>