ti814x: Fix illegal use of FP ops in clock_ti814x.c

The function pll_sigma_delta_val uses "float" data which is not correct.
The exact "why" of this mangling is lost to history, but this changes us
to equivalent non-FP math to get the same results.

Reported-by: Wolfgang Denk <wd@denx.de>
Acked-by: Matt Porter <mporter@linaro.org>