staging: rtl8712: rename function

Rename r8712_get_ndis_wlan_bssid_ex_sz() to r8712_get_wlan_bssid_ex_sz(),
which corresponds to the struct whose size it measures.

Signed-off-by: Joshua Clayton <stillcompiling@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>