net: vlan: Fixed signedness in vlan_group_prealloc_vid()

After commit d018684 ("net: vlan: Avoid using BUG() in
vlan_proto_idx()"), vlan_proto_idx() was changed to return a signed
integer, however one of its called: vlan_group_prealloc_vid() was still
using an unsigned integer for its return value, fix that.

Fixes: d018684 ("net: vlan: Avoid using BUG() in vlan_proto_idx()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/8021q/vlan.c

@@ -51,8 +51,9 @@ static int vlan_group_prealloc_vid(struct vlan_group *vg,
 				   __be16 vlan_proto, u16 vlan_id)
 {
 	struct net_device **array;
-	unsigned int pidx, vidx;
+	unsigned int vidx;
 	unsigned int size;
+	int pidx;
 
 	ASSERT_RTNL();