iovmm is erroneously using sg_dma_len with unmapped (DMA API-wise)
SG entries, and will break if CONFIG_NEED_SG_DMA_LENGTH is enabled.

Fix that by using sg->length instead.
Reported-by: NRussell King <linux@arm.linux.org.uk>
Tested-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NOhad Ben-Cohen <ohad@wizery.com>