metag: Only define atomic_dec_if_positive conditionally

The definition of atomic_dec_if_positive() assumes that
atomic_sub_if_positive() exists, which is only the case if
metag specific atomics are used. This results in the following
build error when trying to build metag1_defconfig.

kernel/ucount.c: In function 'dec_ucount':
kernel/ucount.c:211: error:
	implicit declaration of function 'atomic_sub_if_positive'

Moving the definition of atomic_dec_if_positive() into the metag
conditional code fixes the problem.

Fixes: 6006c0d8 ("metag: Atomics, locks and bitops")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: <stable@vger.kernel.org> # 3.9.x-