ixgbe: fix setting of TXDCTL.WTRHESH when ITR is set to 0 and no BQL

This patch consolidates the logic behind dynamically setting TXDCTL.WTHRESH
depending on interrupt throttle rate (ITR) setting regardless of BQL.

Previously TXDCTL.WTHRESH was dynamically being set only with BQL being
enabled, but we have to set it regardless of BQL when ITR is low to avoid
Tx stalls/hangs.

CC: John Greene <jogreene@redhat.com>
Reported by: Masayuki Gouji <gouji.masayuki@jp.fujitsu.com>
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>