i2c: designware: Move clk_freq into struct dw_i2c_dev

I2c designware controller operate speed is configured in the register
IC_CON. Previously the operate speed is determined by a local variable
clk_freq. This patch will move the local variable clk_freq into struct
dw_i2c_dev. This change will ease the set and get of the clk_freq.

Signed-off-by: Weifeng Voon <weifeng.voon@intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>