i2c: designware: Switch over to i2c_freq_mode_string()

Use generic i2c_freq_mode_string() helper to print chosen bus speed.

Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>