i2c: rk3x: enable clock before getting rate

clk_get_rate() is documented as requiring the clock to be enabled.
Ensure that the bus clock is enabled before calling clk_get_rate() in
rk3x_i2c_probe() to satisfy this requirement.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>