ixgbe: always set header length in SRRCTL

As per the documentation for 82599 in order to support hardware RSC the
header size must be set.  This is only currently done for packet split
mode.  This patch sets the header buffer length for all modes.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>