staging: wilc1000: rename pu8ssid in struct hidden_net_info

This patch renames pu8ssid to ssid that is a member of struct
hidden_net_info. There is no need to use pu8 prefix to show data type so
just remove it.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>