arcmsr(4) changed to use MSI if supported

[Sascha Wildner]

I have changed the arcmsr(4) driver (for Areca RAID controllers) to use  
MSI if it is supported.

It works fine here with my 1210 in a Shuttle box.

If you are using an Areca controller on DragonFly, please give it a try  
(the commit is fb8c9539e80131a1fe791e958dae967b2648aef4) and report any  
issues you find.

Thanks,
Sascha