staging: rtl8188eu: Fix incorrect response to SIOCGIWESSID

When not associated with an AP, wifi device drivers should respond to the
SIOCGIWESSID ioctl with a zero-length string for the SSID, which is the
behavior expected by dhcpcd.

Currently, this driver returns an error code (-1) from the ioctl call,
which causes dhcpcd to assume that the device is not a wireless interface
and therefore it fails to work correctly with it thereafter.

This problem was reported and tested at
https://github.com/lwfinger/rtl8188eu/issues/234

.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>