i2c: Use str_read_write() helper

Remove hard-coded strings by using the str_read_write() helper.

Signed-off-by: Yumeng Fang <fang.yumeng@zte.com.cn>
Signed-off-by: Yunjian Long <long.yunjian@zte.com.cn>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>