mmc: renesas_sdhi_internal_dmac: remove superfluous WARN

The WARN can never trigger because we limited the max_seg number in
renesas_sdhi_of_data already. Remove it and update the comment.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>