usb: dwc2: Simplify wait_for_chhltd(), remove ignore_ack

A transfer is completed if the XFERCOMP flag is set, irrespective of the
ACK flag. BULK OUT transfers to some HS devices complete without having
the ACK flag set, which signal the devices has responded with an NYET
to the transfer (PING protocol).
The new behaviour matches the Linux kernel minus any PING protocol.

Also see 5966defabdcc (usb: dwc2: fix bulk transfers)

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>