media: cx25821: Fix a bug when reallocating some dma memory

This function looks like a realloc.

However, if 'risc->cpu != NULL', the memory will be freed, but never
reallocated with the bigger 'size'.
Explicitly set 'risc->cpu' to NULL, so that the reallocation is
correctly performed a few lines below.

[hverkuil: NULL != risc->cpu -> risc->cpu]

Fixes: 5ede94c7 ("[media] cx25821: remove bogus btcx_risc dependency)
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/media/pci/cx25821/cx25821-core.c

@@ -976,8 +976,10 @@ int cx25821_riscmem_alloc(struct pci_dev *pci,
 	__le32 *cpu;
 	dma_addr_t dma = 0;
 
-	if (NULL != risc->cpu && risc->size < size)
+	if (risc->cpu && risc->size < size) {
 		pci_free_consistent(pci, risc->size, risc->cpu, risc->dma);
+		risc->cpu = NULL;
+	}
 	if (NULL == risc->cpu) {
 		cpu = pci_zalloc_consistent(pci, size, &dma);
 		if (NULL == cpu)