usb: gadget: zero: fix format string warnings

Fixed the following warnings (reported by cppcheck):
[drivers/usb/gadget/function/f_sourcesink.c:1217]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1261]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1305]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1349]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1393]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1437]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1476]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1520]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1564]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.
[drivers/usb/gadget/function/f_sourcesink.c:1608]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'

Reviewed-by: Amit Virdi <amit.virdi@st.com>
Signed-off-by: Asaf Vertz <asaf.vertz@tandemg.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>