SDEI: Fix type of register count

Register count is currently declared as unsigned, where as there are
asserts in place to check it being negative during unregister. These are
flagged as never being true.

Change-Id: I34f00f0ac5bf88205791e9c1298a175dababe7c8
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>

include/services/sdei.h

@@ -154,7 +154,7 @@ typedef struct sdei_ev_map {
 	int32_t ev_num;		/* Event number */
 	unsigned int intr;	/* Physical interrupt number for a bound map */
 	unsigned int map_flags;	/* Mapping flags, see SDEI_MAPF_* */
-	unsigned int reg_count;	/* Registration count */
+	int reg_count;		/* Registration count */
 	spinlock_t lock;	/* Per-event lock */
 } sdei_ev_map_t;