WifiHal: populate llstats peer info with correct offset

llstats peerinfo has varying wifi rate stats. Since
llstats is a linear buffer with interface and peer
stats, we should include wifi rate stats length
while computing offset of next peer

Bug: 191419302
Test: Regression Test
Change-Id: Icc0395cf290268f4c2c96031389fae36ac48d47c