spi: dw: Force set K210 fifo length to 31

The Canaan Kendryte K210 SoC DW apb_ssi v4 spi controller is documented
to have a 32 word deep TX and RX FIFO, which spi_hw_init() detects.
However, when the RX FIFO is filled up to 32 entries (RXFLR = 32), an
RX FIFO overrun error occurs. Avoid this problem by force setting
fifo_len to 31.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>