spi: sirf: fix word width configuration

commit 8c328a26 ("spi: sirf: Avoid duplicate code in various
bits_per_word cases") is wrong in setting data width register of
fifo is not right, it should use sspi->word_width >> 1 to set
related bits. According to hardware spec, the mapping between
register value and data width:
0 - byte
1 - WORD
2 - DWORD

Fixes: 8c328a26

 ("spi: sirf: Avoid duplicate code in various bits_per_word cases") is wrong in setting data width register of
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>
Cc: stable@vger.kernel.org