imx6: fix legacy IRQ support (4.19)

imx6: fix legacy IRQ support (4.19)

The PCIe DWC host controller is now using MSI
(Message-signaled-interrupts) by default.

While ath9k itself does support MSI here, a lot of wlan adapters do not.
Avoid non-functioning cards by simply continue to disable MSI for now.

This can be done by appending "pci=nomsi" to the boot cmdline.

Also an extra fix needs to be backported which avoids MSI initialization
which prevented legacy IRQ's init from taking over.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>