net: dwc_eth_qos: Fix hang when freeing packet after stop

net: dwc_eth_qos: Fix hang when freeing packet after stop

If eqos_free_pkt() is called after eqos_stop(), eqos_stop_resets() will
have been called already. This may prevent accessing the MMIO space to
update the RX descriptor tail pointer, so we must skip the descriptor
maintenance logic. This is okay because the descriptors and tail pointer
will all be rewritten anyway during the next call to eqos_start().

This hang was observed after a failed TFTP transaction:

  eqos_recv(dev=000000047fb57330, flags=1):
  eqos_recv: *packetp=000000c3ffb5c080, length=151

  TFTP error: 'file <FILE> not found for <IP>' (1)
  Not retrying...
  eqos_stop(dev=000000047fb57330):
  eqos_stop: OK
  eqos_free_pkt(packet=000000c3ffb5c080, length=151)
  <HANG>

Fixes: ba4dfef1469f ("net: add driver for Synopsys Ethernet QoS device")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>