staging: vt6655: check too restrictive (off by one)

key_len == MAX_KEY_LEN is valid but we return an error.

Introduced-by: 6b7200fe ('Staging: vt6655: memory corruption in check in wpa_set_wpadev()')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>