qtnfmac: cleanup and unify command error handling

Unify command error handling using qtnf_cmd_resp_result_decode
function. Do not duplicate error messages in command handlers
and cfg80211 callbacks: report 'cmd exec fail' only on control
path internal failure. Remove redundant 'unlikely' macros.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>