omap: iovmm: s/sg_dma_len(sg)/sg->length/

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: Russell King <linux@arm.linux.org.uk>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>