[entropy_src] Fix ENUMASSIGN error in testbench
[entropy_src] Fix ENUMASSIGN error in testbench

Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@opentitan.org>

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