sparc64: vio: Kill BUILD_BUG_ON() in vio_dring_avail().

GCC can't see the 'constant' properly as computed by
is_power_of_2() etc.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/include/asm/vio.h

@@ -258,8 +258,6 @@ static inline void *vio_dring_entry(struct vio_dring_state *dr,
 static inline u32 vio_dring_avail(struct vio_dring_state *dr,
 				  unsigned int ring_size)
 {
-	BUILD_BUG_ON(!is_power_of_2(ring_size));
-
 	return (dr->pending -
 		((dr->prod - dr->cons) & (ring_size - 1)));
 }