qca_spi: Introduce QCASPI_RX_MAX_FRAMES

Currently qca_spi reserves enough space for 4 complete Ethernet over SPI
frames in the receive buffer. Unfortunately this is hidden under a magic
number. So replace it with a more self explaining define.

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>