i2c: designware: Call i2c_dw_clk_rate() only when calculating timings

There are platforms which don't provide input clock rate but provide
I2C timing parameters. Commit 3bd4f277 ("i2c: designware: Call
i2c_dw_clk_rate() only once in i2c_dw_init_master()") causes needless
warning during probe on those platforms since i2c_dw_clk_rate(), which
causes the warning when input clock is unknown, is called even when
there is no need to calculate timing parameters.

Fixes: 3bd4f277

 ("i2c: designware: Call i2c_dw_clk_rate() only once in i2c_dw_init_master()")
Reported-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: <stable@vger.kernel.org> # 4.19
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>