Bug 871475 - 3/6: intantiate additional rmnet nics.

Create <MAX_NET_CLIENTS> additional NIC instances when radio is
enabled. These NICs will have the name "rmnet<N>".

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 4/6: support link down for SMC91C111

Original SMC91C111 doesn't support link up/down reporting (EPHSR).
This patch implement it and one may call |do_set_link| to change the
link status.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 5/6: link up/down in AT+CGACT handler.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>
Bug 871475 - 6/6: implement AT+CGCONTRDP.

Signed-off-by: You-Sheng Yang (楊有勝) <vicamo@gmail.com>