i2c: designware: rename i2c_dw_plat_prepare_clk to i2c_dw_prepare_clk

For consistency with the rest of the file rename function and parameter to
be consistent with the reset of the common file.

Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Phil Reid <preid@electromag.com.au>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>