[dv/flash_ctrl] Fix xcelium coverage error
[dv/flash_ctrl] Fix xcelium coverage error

This PR fixes xcelium coverage error by setting the correct name to
flash_ctrl DV tb.

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

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