mtd: gpmi-nand: fix read page when reading to vmalloced area

The gpmi-nand driver uses virt_addr_valid() to check whether a buffer
is suitable for dma. If it's not, a driver allocated buffer is used
instead. Then after a page read the driver allocated buffer must be
copied to the user supplied buffer. This does not happen since commit
7725cc85.

This patch fixes the issue. The bug is encountered with UBI which uses a
vmalloced buffer for the volume table.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by:  <snijsure@grid-net.com>
Acked-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>