ixgbe: fix default setting of TXDCTL.WTHRESH

The q_vector->itr check in ixgbe_configure_tx_ring() was done prior to it
being set, which resulted in TXDCTL.WTHRESH always being set to 1 on driver
load, while consequent resets would set it to 8.

This patch moves the setting of q_vector->itr in ixgbe_alloc_q_vector() to
make sure that TXDCTL.WTHRESH is set to 8 by default.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>