ion: convert sg_dma_len(sg) to sg->length

ion is always dealing with the allocation and not the mapping,
so it should always be using sg->length and not sg->dma_length.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>