octeontx2-af: Fix return of uninitialized variable err

Currently the variable err may be uninitialized if several of the if
statements are not executed in function nix_tx_vtag_decfg and a garbage
value in err is returned.  Fix this by initialized ret at the start of
the function.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: 9a946de ("octeontx2-af: Modify nix_vtag_cfg mailbox to support TX VTAG entries")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20201118132502.461098-1-colin.king@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/marvell/octeontx2/af/rvu_nix.c

@@ -2085,7 +2085,7 @@ static int nix_tx_vtag_decfg(struct rvu *rvu, int blkaddr,
 	u16 pcifunc = req->hdr.pcifunc;
 	int idx0 = req->tx.vtag0_idx;
 	int idx1 = req->tx.vtag1_idx;
-	int err;
+	int err = 0;
 
 	if (req->tx.free_vtag0 && req->tx.free_vtag1)
 		if (vlan->entry2pfvf_map[idx0] != pcifunc ||