dt-bindings: nfc: ti,trf7970a: fix usage example

The TRF7970A is a SPI device, not I2C.

Signed-off-by: Tobias Schramm <t.schramm@manjaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>