ixl: remove unnecessary limitations related to netmap

[vmaffione]

Summary

This patch is for stable/11.
Netmap supports the case where TX rings and RX rings have different size.
Remove unnecessary limitations related to netmap support, making the code simpler.
Also, check that the value of the hw head index written back from the NIC is valid.

It looks like the value read is invalid for some reason... see https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=230465

[vmaffione]

Sanitize the value of the hw head index read at the end of the TX ring.

[krzysztof.galazka_intel.com]

This limitation came from our out-of-tree driver, where we had to keep backwards compatibility, but there is no reason to keep it in the in-kernel version. Thanks Vincenzo for the patch. It looks good to me but let's wait for Eric to have a look too.

[vmaffione]

No problems!
Actually some people is experiencing problems when using netmap on ixl (see related PR), so I would like to understand what is going on...

[erj]

Having an invalid value for the head writeback value is problematic...I'll check out the PR.

[vmaffione]

Thanks!
It looks like reading the the HEAD of the first TX ring works, while the other TX rings don't...