ixgbe: move Tx ring configuration into a separate function

This patch moves the Tx ring configuration into a separate function.  In
addition the function drops the setting of the head writeback RO bit since
head writeback is no longer used within ixgbe.

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>