This enables MLGRU for Linux 5.15

Difference in power consumption between ondemand and performance
governors is negligible (at least on our boards) while ondemand has
noticeable lag when starting transmissions. Performance was also default
governor in previously shipped kernel, so let's stick to it.

We decided that we are not going to release Turris OS 6.0.2 RC2 with
more changes as we would like to release at least something this week to
improve the current behavior of Rainbow, etc.

So, changes added into RC2 are moved to Turris OS 6.0.3

This patch is required only for the latest Linux backports, but we are
not using it, so we can drop it. Reference:
685634ce ("patches/openwrt: 5.15:
mac80211: downgrade")

This reverts commit 838a2844.

Unfortunately, the latest Linux backports does not work for us as there
are some issues in ath10k and mt76, thus Wi-Fi is not working as we
would like.

I need to revert this as in causes one issue in Jenkins. We are building
three targets on one slave and when it is running at the same time, it
usually fails with:

NOTE: Remote head changed. Local mirror removed for re-clone.
Cloning into bare repository '/home/foo/turris-build/git_mirror/openwrt'...
fatal: '/home/foo/turris-build/git_mirror/openwrt' does not appear to be a git repository
fatal: Could not read from remote repository.

It helps to retrigger manually the job, but thats not wanted behaviour right now.

This reverts commit 36b2b5a9.

Fixes:
build/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/mt76-2022-10-01-72b87836/mt7915/init.c:330:37: error: 'IEEE80211_MAX_AMPDU_BUF' undeclared (first use in this function); did you mean 'IEEE80211_MIN_AMPDU_BUF'?
  hw->max_rx_aggregation_subframes = IEEE80211_MAX_AMPDU_BUF;
                                     ^~~~~~~~~~~~~~~~~~~~~~~
                                     IEEE80211_MIN_AMPDU_BUF

This fixes issue within different GNU Linux distributions:
- Ubuntu and Debian starts with the number(s) and then with alphabet
- OpenSuse does it quite opposite

So, this fixes issue for both GNU Linux distributions