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: 291ab06 ("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>

Author: lategoodbye

drivers/net/ethernet/qualcomm/qca_debug.c

@@ -30,6 +30,8 @@
 
 #define QCASPI_MAX_REGS 0x20
 
+#define QCASPI_RX_MAX_FRAMES 4
+
 static const u16 qcaspi_spi_regs[] = {
 	SPI_REG_BFR_SIZE,
 	SPI_REG_WRBUF_SPC_AVA,
@@ -252,9 +254,9 @@ qcaspi_get_ringparam(struct net_device *dev, struct ethtool_ringparam *ring,
 {
 	struct qcaspi *qca = netdev_priv(dev);
 
-	ring->rx_max_pending = 4;
+	ring->rx_max_pending = QCASPI_RX_MAX_FRAMES;
 	ring->tx_max_pending = TX_RING_MAX_LEN;
-	ring->rx_pending = 4;
+	ring->rx_pending = QCASPI_RX_MAX_FRAMES;
 	ring->tx_pending = qca->txr.count;
 }
 
@@ -265,7 +267,7 @@ qcaspi_set_ringparam(struct net_device *dev, struct ethtool_ringparam *ring,
 {
 	struct qcaspi *qca = netdev_priv(dev);
 
-	if ((ring->rx_pending) ||
+	if (ring->rx_pending != QCASPI_RX_MAX_FRAMES ||
 	    (ring->rx_mini_pending) ||
 	    (ring->rx_jumbo_pending))
 		return -EINVAL;