[fpv] Fix some assumptions in prim_count
[fpv] Fix some assumptions in prim_count

This PR fixes some assumptions in prim_count for cross mode.
1). The down counter underflow assumption used `%` that could be very
complex for FPV. I slightly change the assumption to avoid `%`.
2). Underflow and overflow assumptions should apply to all cross
counters because they have up and down counters.
3). ASSERT_INIT for FPV is only used for parameters and constants, thus
does not apply to the wire. Excluded that assertion check for simulation
only.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@opentitan.org>

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