qcacmn: dp: Replace explicit comparison to NULL

Per the Linux Kernel coding style, as enforced by the kernel
checkpatch script, pointers should not be explicitly compared to
NULL. Therefore within dp replace any such comparisons with logical
operations performed on the pointer itself.

Change-Id: I61f3adab1208682d36235421f44a048e35df346d
CRs-Fixed: 2418258

dp/wifi3.0/dp_rx.h

@@ -398,7 +398,7 @@ void dp_rx_add_to_free_desc_list(union dp_rx_desc_list_elem_t **head,
 
 	((union dp_rx_desc_list_elem_t *)new)->next = *head;
 	*head = (union dp_rx_desc_list_elem_t *)new;
-	if (*tail == NULL)
+	if (!*tail)
 		*tail = *head;
 
 }