i2c: sh_mobile: fix leak when using DMA bounce buffer

We only freed the bounce buffer after successful DMA, missing the cases
where DMA setup may have gone wrong. Use a better location which always
gets called after each message and use 'stop_after_dma' as a flag for a
successful transfer.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>