ice: do not disable Tx queues twice in ice_down()

ice_down() clears QINT_TQCTL_CAUSE_ENA_M bit twice, which is not
necessary. First clearing happens in ice_vsi_dis_irq() and second in
ice_vsi_stop_tx_ring() - remove the first one.

While at it, make ice_vsi_dis_irq() static as ice_down() is the only
current caller of it.

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>