ixgbevf: Resolve warnings for -Wimplicit-fallthrough

Additions to gcc 7 now warn whenever a switch statement falls through
implicitly.  This patch adds explicit fall through comments to address the
following warnings:

drivers/net/ethernet/intel/ixgbevf/vf.c: In function ‘ixgbevf_get_reta_locked’:
drivers/net/ethernet/intel/ixgbevf/vf.c:336:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (hw->mac.type < ixgbe_mac_X550_vf)
      ^
drivers/net/ethernet/intel/ixgbevf/vf.c:338:2: note: here
  default:
  ^~~~~~~
drivers/net/ethernet/intel/ixgbevf/vf.c: In function ‘ixgbevf_get_rss_key_locked’:
drivers/net/ethernet/intel/ixgbevf/vf.c:402:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (hw->mac.type < ixgbe_mac_X550_vf)
      ^
drivers/net/ethernet/intel/ixgbevf/vf.c:404:2: note: here
  default:
  ^~~~~~~

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>