dt-binding: cdns,usb3: Fix cdns,on-chip-buff-size type

In cdns3-gadget.c, 'cdns,on-chip-buff-size' was read using
device_property_read_u16(). It resulted in 0 if a 32bit value was used
in dts. This commit fixes the dt binding doc to declare it as u16.

Cc: stable@vger.kernel.org
Fixes: 68989fe1

 ("dt-bindings: usb: Convert cdns-usb3.txt to YAML schema")
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>