mlxsw: spectrum: fix uninitialized value in err

In the unlikely event that mfc->mfc_un.res.ttls[i] is 255 for all
values of i from 0 to MAXIVS-1, the err is not set at all and hence
has a garbage value on the error return at the end of the function,
so initialize it to 0.  Also, the error return check on err and goto
to err: inside the for loop makes it impossible for err to be zero
at the end of the for loop, so we can remove the redundant err check
at the end of the loop.

Detected by CoverityScan CID#1457207 ("Unitialized scalar value")

Fixes: c011ec1b

 ("mlxsw: spectrum: Add the multicast routing offloading logic")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Yotam Gigi <yotamg@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>