qca_spi: Improve calculation of RX buffer size

There are two points with the calculation of RX buffer size which are
not optimal:
1. dev->mtu is a mutual parameter and it's currently initialized with
   QCAFRM_MAX_MTU. But for RX buffer size calculation we always need the
   maximum possible MTU. So better use the define directly.
2. This magic number 4 represent the hardware generated frame length
   which is specific to SPI. We better replace this with the suitable
   define.

There is no functional change.

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/qca_spi.c

@@ -828,8 +828,8 @@ qcaspi_netdev_init(struct net_device *dev)
 	qca->clkspeed = qcaspi_clkspeed;
 	qca->burst_len = qcaspi_burst_len;
 	qca->spi_thread = NULL;
-	qca->buffer_size = (dev->mtu + VLAN_ETH_HLEN + QCAFRM_HEADER_LEN +
-		QCAFRM_FOOTER_LEN + 4) * QCASPI_RX_MAX_FRAMES;
+	qca->buffer_size = (QCAFRM_MAX_MTU + VLAN_ETH_HLEN + QCAFRM_HEADER_LEN +
+		QCAFRM_FOOTER_LEN + QCASPI_HW_PKT_LEN) * QCASPI_RX_MAX_FRAMES;
 
 	memset(&qca->stats, 0, sizeof(struct qcaspi_stats));