spi: sirf: refactor spi transfer functions

split sirfsoc_spi_transfer function into 3 sub-functions:
spi_sirfsoc_cmd_transfer, spi_sirfsoc_pio_transfer and
spi_sirfsoc_dma_transfer.
Signed-off-by: Qipan Li <Qipan.Li@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Mark Brown <broonie@linaro.org>