i2c: rcar: avoid unused ret variable

Fix smatch warning:
drivers/i2c/busses/i2c-rcar.c:628 \
  rcar_i2c_request_dma_chan() warn: unused return: ret = PTR_ERR()

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>