bridge: mdb: avoid uninitialized variable warning

A recent change to the mdb code confused the compiler to the point
where it did not realize that the port-group returned from
br_mdb_add_group() is always valid when the function returns a nonzero
return value, so we get a spurious warning:

net/bridge/br_mdb.c: In function 'br_mdb_add':
net/bridge/br_mdb.c:542:4: error: 'pg' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    __br_mdb_notify(dev, entry, RTM_NEWMDB, pg);

Slightly rearranging the code in br_mdb_add_group() makes the problem
go away, as gcc is clever enough to see that both functions check
for 'ret != 0'.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 9e8430f8 ("bridge: mdb: Passing the port-group pointer to br_mdb module")
Signed-off-by: David S. Miller <davem@davemloft.net>