hmi: Fix clearing HMER on debug trigger

In the recent patch:
  eddff9b hmi: Clear unknown debug trigger

I rebased the code from an older skiboot before the HMI rework. When I
did this, I missed the handled flag. Without this the HMER is not
cleared properly and the HMI keeps happening.

This properly sets the handled flag and hence clears the HMER bit.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

core/hmi.c

@@ -1259,6 +1259,7 @@ static int handle_hmi_exception(uint64_t hmer, struct OpalHMIEvent *hmi_evt,
 		}
 	}
 	if (hmer & SPR_HMER_TRIG_FIR_HMI) {
+		handled |= SPR_HMER_TRIG_FIR_HMI;
 		hmer &= ~SPR_HMER_TRIG_FIR_HMI;
 
 		hmi_print_debug("Clearing unknown debug trigger", hmer);