staging: rtl8712: simplify size calculation

replace item-by-item  size calculation of a struct
with the size of the struct.

This 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>