[FS#1245] [Netgear R7800] 2.4GHz wifi stuck at 20MHz (40 MHz channel width doesn't work)

[LEDE Bugs]

A new Flyspray task has been opened.  Details are below. 

User who did this - Mirco Ebasta (Aye24) 

Attached to Project - LEDE Project
Summary - [Netgear R7800] 2.4GHz wifi stuck at 20MHz (40 MHz channel width doesn't work)
Task Type - Bug Report
Category - Base system
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Medium
Priority - Very Low
Reported Version - All
Due in Version - Undecided
Due Date - Undecided
Details - Netgear Nighthawk X4S R7800
LEDE Reboot SNAPSHOT r5442-b0b289ea45 / LuCI Master (git-17.328.04231-802d5b6)
Kernel Version	4.9.65

On 5GHz the 80MHz width works fine, 
but on 2.4GHz the 40MHz width doesn't work,
it's always stuck on 20MHz.

More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1245