i2c: bcm-kona: fix error return code in bcm_kona_i2c_probe()

Fix to return a negative error code from the bus speed parse
error handling case instead of 0.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Tim Kryger <tim.kryger@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>