i2c: designware: fix i2c_dw_clk_rate() return size to be u32

Make i2c_dw_clk_rate() to return u32 instead of unsigned long, as the
function return the value of get_clk_rate_khz() which returns u32.

Fixes: b33af11d ("i2c: designware: Do not require clock when SSCN and FFCN are provided")
Signed-off-by: Hanna Hawa <hhhawa@amazon.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>