xen-netback: remove redundant assignment to err

The variable err is assigned with the value -ENOMEM that is never
read and it is re-assigned a new value later on.  The assignment is
redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>