[fpv/pinmux] Fix assertions for issue #9850
[fpv/pinmux] Fix assertions for issue #9850

As issue #9850 clarifed, it is a misunderstanding on my verification
side, I updated the failed assertions according to the comment
from #9850.

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

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan