bnxt_en: More robust SRIOV cleanup sequence.

Instead of always calling pci_sriov_disable() in remove_one(),
the driver should detect whether VFs are currently assigned
to the VMs. If the VFs are active in VMs, then it should not
disable SRIOV as it is catastrophic to the VMs. Instead,
it just leaves the VFs alone and continues to unload the PF.
The user can then cleanup the VMs even after the PF driver
has been unloaded.

Signed-off-by: Jeffrey Huang <huangjw@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>