clk: stratix10: use do_div() for 64-bit calculation

do_div() macro to perform u64 division and guards against overflow if
the result is too large for the unsigned long return type.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Link: https://lkml.kernel.org/r/20200114160726.19771-1-dinguyen@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>