ixgbevf: Fix errors in retrieving RETA and RSS from PF

Mailbox support for getting RETA and RSS is available for only 82599 and
x540; a previous patch reversed the logic and these adapters were
returning not supported.

Also, the NACK check in ixgbevf_get_rss_key_locked() was checking for the
command IXGBE_VF_GET_RETA instead of IXGBE_VF_GET_RSS_KEY.

This patch corrects both issues by correcting the logic and checking for
the right command.

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>