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: 9a946def

 ("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>