usb: gadget: zero: Fix warning generated by kbuild

The kbuild test bot generated the warning:
	drivers/usb/gadget/function/f_sourcesink.c:1498: warning: comparison is
	always false due to limited range of data type

This patch fixes it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Amit Virdi <amit.virdi@st.com>
CC: Felipe Balbi <balbi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>