staging: wilc1000: fix NULL dereference inside wilc_scan()

Added NULL check before accessing 'hidden_net' pointer inside
wilc_scan() to fix the issue found by static code checker.

Fixes: 8f1a0ac1 ("staging: wilc1000: handle scan operation callback from cfg80211 context")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>