tests: unit: crc: use `zassert_equal` for cleaner assertions

Replace `zassert` with `zassert_equal` in CRC8-CCITT and CRC8-ROHC
test cases, as `zassert_equal` is more appropriate for comparing
actual and expected values.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>