i40e: add required steps in Tx queue management

i40e: add required steps in Tx queue management

Hardware specification changed recently which requires to set
or clear TX queue disable flags before actually enabling or
disabling a specific TX queue. 'QTX_HEAD' register needs to
be cleared before setting the QENA_REQ flag.

Signed-off-by: Helin Zhang <helin.zhang@intel.com>
Acked-by: Jijiang Liu <jijiang.liu@intel.com>
Acked-by: Jing Chen <jing.d.chen@intel.com>
Tested-by: Min Cao <min.cao@intel.com>