[prim] Fix missing case from prim_reg_cdc_arb assert
[prim] Fix missing case from prim_reg_cdc_arb assert

- should fix #14526

- The assertion forgot to account for the case where a hardware update
  request was received while there is already an ongoing handshake
  either from software or from a previous hardware update.

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

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