i40evf: Don't Panic

Under some circumstances the driver remove function may be called before
the driver is fully initialized. So we can't assume that we know where
our towel is at, or that all of the data structures are initialized.

To ensure that we don't panic, check that the vsi_res pointer is valid
before dereferencing it. Then drink beer and eat peanuts.

Change-ID: If697b4db57348e39f9538793e16aa755e3e1af03
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>