[media] vb2: Fix allocation size of dma_parms

When allocating memory to hold the device dma parameters in
vb2_dma_contig_set_max_seg_size(), the requested size is by mistake only
the size of a pointer. Request the correct size instead.

Fixes: 3f033969 ("media: vb2-dma-contig: add helper for setting dma max seg size")

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>