mtd: mtdram: properly handle the phys argument in the point method

When the phys pointer is non null, the point method is expected to return
the physical address for the pointed area. In the case of the mtdram
driver we have to retrieve the physical address for the corresponding
vmalloc area. However, there is no guarantee that the vmalloc area is
made of physically contiguous pages. In that case we simply limit retlen
to the actually contiguous pages.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Reviewed-by: Richard Weinberger <richard@nod.at>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>