sfc: remove duplicate up_write on VF filter_sem

Somehow two copies of the line 'up_write(&vf->efx->filter_sem);' got into
 efx_ef10_sriov_set_vf_vlan().  This would put the mutex in a bad state and
 cause all subsequent down attempts to hang.

Fixes: 671b53ee ("sfc: Ensure down_write(&filter_sem) and up_write() are matched before calling efx_net_open()")
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>