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>