mtd: gpmi: fix raw_buffer pointer double free issue

fix the raw_buffer pointer double free issue found by coverify.

CID 18344 (#2 of 2): Double free (USE_AFTER_FREE)
3. double_free: Calling gpmi_alloc_dma_buffer frees pointer
this->raw_buffer which has already been freed

Signed-off-by: Han Xu <han.xu@nxp.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>

drivers/mtd/nand/gpmi-nand/gpmi-nand.c

@@ -797,6 +797,7 @@ static void gpmi_free_dma_buffer(struct gpmi_nand_data *this)
 
 	this->cmd_buffer	= NULL;
 	this->data_buffer_dma	= NULL;
+	this->raw_buffer	= NULL;
 	this->page_buffer_virt	= NULL;
 	this->page_buffer_size	=  0;
 }