net: vlan: Fixed signedness in vlan_group_prealloc_vid()

After commit d0186842 ("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: d0186842

 ("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>