qca_debug: Fix ethtool -G iface tx behavior

qca_debug: Fix ethtool -G iface tx behavior

After calling ethtool -g it was not possible to adjust the TX ring
size again:

  # ethtool -g eth1
  Ring parameters for eth1:
  Pre-set maximums:
  RX:		4
  RX Mini:	n/a
  RX Jumbo:	n/a
  TX:		10
  Current hardware settings:
  RX:		4
  RX Mini:	n/a
  RX Jumbo:	n/a
  TX:		10
  # ethtool -G eth1 tx 8
  netlink error: Invalid argument

The reason for this is that the readonly setting rx_pending get
initialized and after that the range check in qcaspi_set_ringparam()
fails regardless of the provided parameter. So fix this by accepting
the exposed RX defaults. Instead of adding another magic number
better use a new define here.

Fixes: 291ab06ecf67 ("net: qualcomm: new Ethernet over SPI driver for QCA7000")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Link: https://lore.kernel.org/r/20231206141222.52029-3-wahrenst@gmx.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>