i40e: stop flow director on shutdown

In some cases, the hardware would continue to try to access the FDIR
ring after entering D3Hot state, which would cause either PCIe errors or
NMIs, depending upon system configuration.

Explicitly stop FDIR in our shutdown routine to eliminate this
possibility.

Change-ID: I1bd9fc7fd8f151fe24cad132ac9adddab923e3af
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>