firewire: core: typecast from gfp_t to bool more safely

An idr related patch introduced the following sparse warning:
  drivers/firewire/core-cdev.c:488:33: warning: incorrect type in initializer (different base types)
  drivers/firewire/core-cdev.c:488:33:    expected bool [unsigned] [usertype] preload
  drivers/firewire/core-cdev.c:488:33:    got restricted gfp_t
So let's convert from gfp_t bitfield to Boolean explicitly and safely.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>

drivers/firewire/core-cdev.c

@@ -486,7 +486,7 @@ static int ioctl_get_info(struct client *client, union ioctl_arg *arg)
 static int add_client_resource(struct client *client,
 			       struct client_resource *resource, gfp_t gfp_mask)
 {
-	bool preload = gfp_mask & __GFP_WAIT;
+	bool preload = !!(gfp_mask & __GFP_WAIT);
 	unsigned long flags;
 	int ret;