wireless-drivers fixes for 4.18

First set of fixes for 4.18 and for numerous drivers. Something to mention
about is the wcn36xx fix which makes it possible to compile with gcc older than
4.4 (though I'm not sure if we even support those anymore).

qtnfmac

* coverity fix for a new commit in v4.18-rc1

rtlwifi

* fix kernel oops during driver removal

* fix firmware image corruption for rtl8821ae

brcmfmac

* fix crash if there's no firmware image

mwifiex

* a revert and a better fix for a new commit v4.18-rc1

mt7601u

* fix a recent regression about unnecessary warning about avg_rssi

wcn36xx

* convert testmode.c to plain ASCII

ath10k

* fix a firmware crash during bandwidth change
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Merge ath-current from git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git

ath.git fixes for 4.18. Major changes:

wcn36xx

* convert testmode.c to plain ASCII

ath10k

* fix a firmware crash during bandwidth change