net/wan/fsl_ucc_hdlc: allow ucc index up to 7

There is a need to allow higher indexes to be
able to support MPC83xx platforms. (UCC1-UCC8)

Signed-off-by: David Gounaris <david.gounaris@infinera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>