staging: rtl8712: fix buggy size calculation

r8712_get_ndis_wlan_bssid_ex_sz has a "6 * sizeof(unsigned long)"
where the underlying struct has a 6 * unsigned char.
Simplify the calculation by just subtracting the variable part from
the size of the struct.

This also gets rid of a use of typedef NDIS_802_11_RATES_EX

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