clk: axi-clkgen: Round closest in round_rate() and recalc_rate()

To minimize the rounding error round to the closest integer when
calculating the result in the recalc_rate() and set_rate() callbacks.

Also in order to improve precision multiply first and then divide.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>