hw/usb/hcd-xhci: don't assert on NAK when retrying an isoch transfer

The endpoint type in the xHCI endpoint context comes from the guest and
is not checked against the device. A guest can configure the interrupt
IN endpoint of usb-kbd as Isoch IN. The idle HID endpoint NAKs, and as
soon as the transfer goes through the retry path in xhci_kick_epctx()
it hits

  assert(xfer->packet.status != USB_RET_NAK);

No device model NAKs on an isoch endpoint, so this only triggers with a
mismatched endpoint type.

The two retry branches differ only in what they do on NAK: the isoch one
asserts, the other keeps the transfer pending. Merge them.

Fixes: 3d1396842d ("xhci: iso xfer support")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3886
Reported-by: Feifan Qian <bea1e@proton.me>
Cc: qemu-stable@nongnu.org
Signed-off-by: Junjie Cao <junjie.cao@intel.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <44227b05b22064a22f4c135e025ff0b1d838b9e3.1789968699.git.junjie.cao@intel.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
(cherry picked from commit 45684127cdee2d212125fd8d1de8a9c4804ee8bb)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>