While we have a local variable "buf_len" for dev->tx_buf_len, we don't
have such local variable for dev->tx_buf pointer.  While "buf_len" is
restored at first then updated when we start processing a new i2c_msg
(determined by STATUS_WRITE_IN_PROGRESS flag), ->tx_buf is different.

Such inconsistency makes the code slightly hard to follow.
Signed-off-by: NShinya Kuribayashi <shinya.kuribayashi@necel.com>
Acked-by: NBaruch Siach <baruch@tkos.co.il>
Signed-off-by: NBen Dooks <ben-linux@fluff.org>