frv: fix compiler warning from definition of __pmd()

Take into account that the pmd_t type is a array inside a struct, so it
needs two levels of brackets to initialize.  Otherwise, a usage of __pmd
generates a warning:

  include/linux/mm.h:986:2: warning: missing braces around initializer [-Wmissing-braces]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/frv/include/asm/page.h

@@ -34,7 +34,7 @@ typedef struct page *pgtable_t;
 #define pgprot_val(x)	((x).pgprot)
 
 #define __pte(x)	((pte_t) { (x) } )
-#define __pmd(x)	((pmd_t) { (x) } )
+#define __pmd(x)	((pmd_t) { { (x) } } )
 #define __pud(x)	((pud_t) { (x) } )
 #define __pgd(x)	((pgd_t) { (x) } )
 #define __pgprot(x)	((pgprot_t) { (x) } )