spi: sirf: fix word width configuration

spi: sirf: fix word width configuration

commit 8c328a262f ("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: 8c328a262f ("spi: sirf: Avoid duplicate code in various bits_per_word cases") is wrong in setting data width register of
Signed-off-by: Qipan Li <[email protected]>
Signed-off-by: Barry Song <[email protected]>
Signed-off-by: Mark Brown <[email protected]>
Cc: [email protected]