qca_spi: Adjust log of SPI_REG_RDBUF_BYTE_AVA

All known SPI registers of the QCA700x are 16 bit long. So adjust
the formater width accordingly.

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>