ixgbevf: Check for RSS key before setting value

The RSS key is being repopulated every time the interface is brought up
regardless of whether there is an existing value. If the user sets the RSS
key and the interface is brought up (e.g. reset), the user specified RSS
key will be overwritten.

This patch changes the rss_key to a pointer so we can check to see if the
key has been populated and preserve it accordingly.

Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>