timer: sp804: Fix dev_read_addr error check

dev_read_addr returns FDT_ADDR_T_NONE (-1) in case of error
and not 0.

Signed-off-by: Francois Berder <fberder@outlook.fr>

drivers/timer/sp804_timer.c

@@ -44,7 +44,7 @@ static int sp804_clk_of_to_plat(struct udevice *dev)
 	struct sp804_timer_plat *plat = dev_get_plat(dev);
 
 	plat->base = dev_read_addr(dev);
-	if (!plat->base)
+	if (plat->base == FDT_ADDR_T_NONE)
 		return -ENOENT;
 
 	return 0;