The condition "if (need_zero_pkt && zero_len_trb)" was always false
and it caused that TRB for ZLP was not prepared.

Fix causes that after preparing last TRB in TD, the driver prepares
additional TD with ZLP when a ZLP is required.

Cc: <stable@vger.kernel.org>
Fixes: 3d829045 ("usb: cdnsp: cdns3 Add main part of Cadence USBSSP DRD Driver")
Signed-off-by: Pawel Laszczak <pawell@cadence.com>
Link: https://lore.kernel.org/r/20210623072728.41275-1-pawell@gli-login.cadence.comSigned-off-by: Peter Chen <peter.chen@kernel.org>