spi: sirf: add fifo reset/start for cmd transfer

for command mode spi transfer, HW spec requires to do fifo reset work to
clear FIFO status.

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>