niu: Fix missing unwind goto in niu_alloc_channels()

Smatch reports: drivers/net/ethernet/sun/niu.c:4525
	niu_alloc_channels() warn: missing unwind goto?

If niu_rbr_fill() fails, then we are directly returning 'err' without
freeing the channels.

Fix this by changing direct return to a goto 'out_err'.

Fixes: a3138df9

 ("[NIU]: Add Sun Neptune ethernet driver.")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>