[clkmgr] Fix an ENUMASSIGN error in the testbench

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

hw/ip/clkmgr/dv/env/clkmgr_if.sv

diff --git a/hw/ip/clkmgr/dv/env/clkmgr_if.sv b/hw/ip/clkmgr/dv/env/clkmgr_if.sv
index ed0d4ce..8366c59 100644
--- a/hw/ip/clkmgr/dv/env/clkmgr_if.sv
+++ b/hw/ip/clkmgr/dv/env/clkmgr_if.sv
@@ -81,7 +81,7 @@
 
   prim_mubi_pkg::mubi4_t extclk_ctrl_csr_step_down;
   always_comb begin
-     extclk_ctrl_csr_step_down = ~`CLKMGR_HIER.hi_speed_sel_o;
+     extclk_ctrl_csr_step_down = prim_mubi_pkg::mubi4_t'(~`CLKMGR_HIER.hi_speed_sel_o);
   end
 
   prim_mubi_pkg::mubi4_t jitter_enable_csr;