wlan: Rectify the correct size while putting wifi stats

Presently there is an error in the calculation of the size.
The member tSirWifiRateStat is an static array of size 1 and not a
pointer. Thus while incrementing the size for the next instance, a page
fault occurs.

Instead of using numRates, use numRates-1 as the structure already
accounts for the first size.

Change-Id: Ic92179d1eb324f2092887a487f4a32598298a1b3
CRs-Fixed: 2357163