mac80211: do not compute offset from ssn in Rx AMPDU reordering buffer

Currently, frames that go into the reordering buffer are stored at
index ieee80211_sn_sub(sn, tid_rx->ssn) % tid_rx->buf_size.

The offset calculation to the starting sequence number (SSN) is
useless and just adds overhead so simply use sn % tid_rx->buf_size.

This means the reordering buffer will start to be filled somewhere
in the middle (at SSN % buf_size) and continue to get used from
there, but there's no reason to start from the beginning.

Signed-off-by: Karl Beldan <karl.beldan@rivierawaves.com>
[rewrite commit message]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>