i2c: tegra: add spinlock definition comment

Fix checkpatch.pl CHECK as follows:
CHECK: spinlock_t definition without comment
+       spinlock_t xfer_lock;
Signed-off-by: Bitan Biswas <bbiswas@nvidia.com>
Reviewed-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-tegra.c

@@ -269,6 +269,7 @@ struct tegra_i2c_dev {
 	u32 bus_clk_rate;
 	u16 clk_divisor_non_hs_mode;
 	bool is_multimaster_mode;
+	/* xfer_lock: lock to serialize transfer submission and processing */
 	spinlock_t xfer_lock;
 	struct dma_chan *tx_dma_chan;
 	struct dma_chan *rx_dma_chan;