i2c: sirf: fix the typo for setting bitrate to less than 100k

there is a typo before, it makes the final bitrate wrong, this patch fixes
it.

Signed-off-by: Zhiwu Song <Zhiwu.Song@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>