bfdd: Fix overflow possibility with time statements

bfdd: Fix overflow possibility with time statements

If time ( a uint64_t ) is large enough doing division
and subtraction can still lead to situations where
the resulting number is greater than a uint32_t.
Just use uint32_t as an intermediate storage spot.
This is unlikely to every occur in a time frame
I could possibly care about but makes Coverity happy.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>