i2c: rcar: make rcar_i2c_prepare_msg() *void*

rcar_i2c_prepare_msg() always returns 0, so we can make this function return
*void* and thus remove the result check in rcar_i2c_master_xfer().

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>