net/mlx5e: Fix uninitialized variable

There is a potential execution path in which variable *err* is returned
without being properly initialized previously.

Fix this by initializing variable *err* to 0.

Addresses-Coverity-ID: 1472116 ("Uninitialized scalar variable")
Fixes: 0ec13877 ("net/mlx5e: Gather all XDP pre-requisite checks in a single function")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>