ASoC: fsl_spdif: Fix integer overflow when calculating divisor

The calculation code does
u64 = (u32 - u32) * 100000;

The 64 bits are of no help here as the type is casted only after the
multiplication, and therefore the result may overflow, possibly causing
inoptimal or wrong clock setup.

Fix the code to cast before multiplication.

Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>