ixgbe: combine two modifications of TXDCTL into one

In ixgbe_up_complete we were doing a read-modify-write of TXDCTL followed
by another one just a few lines further down.  Instead of performing two
separate read-modify-writes it would make more sense to combine the two
into one.

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>