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>