spi: sirf: add the reset for USP-based SPI

USP-based SPI need a disable and enable, otherwise it doesn't work.
this patch adds it as HW initialization.

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>