rtl8192cu: Call ieee80211_register_hw from rtl_usb_probe

In a previous patch the call to ieee80211_register_hw was moved from the
load firmware callback to the rtl_pci_probe only.
rt8192cu also uses this callback. Currently it doesnt create a wlan%d device.
Fill in the call to ieee80211_register_hw in rtl_usb_probe.

Signed-off-by: Karsten Wiese <fzuuzf@googlemail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>