i2c: imx: enable clk before write to registers

The module clk should be enabled before write to
its register in probe(), and the clk can be disabled
after.

Signed-off-by: Jingchang Lu <b35083@freescale.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>