xen-netback: Remove redundant initialization of variable err

The variable err is being initialized with a value that is never read, it
is being updated immediately afterwards. The assignment is redundant and
can be removed.

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