spi: sirf: fix 'cmd_transfer' function typos

unify 'cmd_transfer' like 'pio_transfer' and 'dma_transfer' as void
function, and also change left_rx_word according to transfer result.

Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@kernel.org>