ixgbe: Remove tail write abstraction and add missing barrier

This change cleans up the tail writes for the ixgbe descriptor queues.  The
current implementation had me confused as I wasn't sure if it was still
making use of the surprise remove logic or not.

It also adds the mmiowb which is needed on ia64, mips, and a couple other
architectures in order to synchronize the MMIO writes with the Tx queue
_xmit_lock spinlock.

Cc: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>