i2c: smbus: kill memory leak on emulated and failed DMA SMBus xfers

If DMA safe memory was allocated, but the subsequent I2C transfer
fails the memory is leaked. Plug this leak.

Fixes: 8a77821e

 ("i2c: smbus: use DMA safe buffers for emulated SMBus transactions")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Cc: stable@kernel.org