i2c: rcar: fix clk_get() error handling

When clk_get() fails, it returns an error code, not a NULL. This patch
fixes such an error handling bug.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>