nand: zynq: Fix driver initialization

nand: zynq: Fix driver initialization

This driver is currently broken, refusing to initialize properly.

The reason is that get_nand_dev_by_index() was being called before
nand_register(), thus returning a pointer into uninitialized memory.
In other words, the struct mtd_info used by the driver is total junk.

Fix it by getting the correct struct mtd_info, via nand_to_mtd()
on the driver's struct nand_chip.

Tested on a custom board, where the CPU is halted without this patch.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>