net/mlx5e: Don't use err uninitialized in mlx5e_attach_decap

Clang warns:

drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:3712:6: warning:
variable 'err' is used uninitialized whenever 'if' condition is false
[-Wsometimes-uninitialized]
        if (IS_ERR(d->pkt_reformat)) {
            ^~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:3718:6: note:
uninitialized use occurs here
        if (err)
            ^~~
drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:3712:2: note: remove the
'if' if its condition is always true
        if (IS_ERR(d->pkt_reformat)) {
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:3670:9: note: initialize
the variable 'err' to silence this warning
        int err;
               ^
                = 0
1 warning generated.

It is not wrong, err is only ever initialized in if statements but this
one is not in one. Initialize err to 0 to fix this.

Fixes: 14e6b038 ("net/mlx5e: Add support for hw decapsulation of MPLS over UDP")
Link: https://github.com/ClangBuiltLinux/linux/issues/1037


Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>