ixgbe: Drop unnecessary addition from ixgbe_set_rx_buffer_len

We still had some code floating around from the old single buffer receive
path.  As a result we were adding VLAN_HLEN to max_frame although the
resultant value was never used.  Since that is the case we can drop this from
the function.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>