ixgbevf: Add legacy suffix to old API mailbox functions

Add legacy suffix to mailbox functions which should be backwards compatible
with older PF drivers. Communication during API negotiation always has to
be done using the earlier implementation.

Signed-off-by: Radoslaw Tyl <radoslawx.tyl@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>