gve: Update mgmt_msix_idx if num_ntfy changes

If we do not get the expected number of vectors from
pci_enable_msix_range, we update priv->num_ntfy_blks but not
priv->mgmt_msix_idx. This patch fixes this so that priv->mgmt_msix_idx
is updated accordingly.

Fixes: f5cedc84

 ("gve: Add transmit and receive support")
Signed-off-by: David Awogbemila <awogbemila@google.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>