nvramtool: Remove the building warning on older gcc

Some older gcc requires the default entry in switch, otherwise
build warning "enumeration value not handled in switch" will come
up.

Change-Id: Ic8ea9960e4aca599e0ea62ec345122c9df57e766
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/1501
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

util/nvramtool/cmos_lowlevel.c

@@ -55,6 +55,7 @@ void select_hal(hal_t hal, void *data)
 			break;
 #endif
 		case HAL_MEMORY:
+		default:
 			current_access = &memory_hal;
 			break;
 	}