arch/unicore32/mm/ioremap.c: return NULL on invalid pfn

__uc32_ioremap_pfn_caller() should return NULL when the pfn is found to be
invalid.

From a recommendation by Guan Xuetao.

Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Cc: Fabian Frederick <fabf@skynet.be>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/unicore32/mm/ioremap.c

@@ -143,11 +143,13 @@ void __iomem *__uc32_ioremap_pfn_caller(unsigned long pfn,
 	/*
 	 * Don't allow RAM to be mapped
 	 */
-	if (pfn_valid(pfn))
+	if (pfn_valid(pfn)) {
 		WARN(1, "BUG: Your driver calls ioremap() on\n"
 			"system memory.  This leads to architecturally\n"
 			"unpredictable behaviour, and ioremap() will fail in\n"
 			"the next kernel release. Please fix your driver.\n");
+		return NULL;
+	}
 
 	type = get_mem_type(mtype);
 	if (!type)