sfc: avoid duplicated code in ef10_sriov

The fail path of efx_ef10_sriov_alloc_vf_vswitching is identical to the
full content of efx_ef10_sriov_free_vf_vswitching, so replace it for a
single call to efx_ef10_sriov_free_vf_vswitching.

Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>