i2c: sprd: Validate the return value of clock initialization

The 'enable' clock of I2C master is required, we should return an error
if failed to get the 'enable' clock, to make sure the I2C driver can be
defer probe if the clock resource is not ready.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>