eth fbnic: Add register self test

eth fbnic: Add register self test

The register test will be used to verify hardware is behaving as expected.

The test itself will have us writing to registers that should have no
side effects due to us resetting after the test has been completed.

While the test is being run the interface should be offline.

This patch counts on the first patch of this series to export netif_open()
and also ensures that the half close calls netif_close() to
avoid deadlock.

Signed-off-by: Mike Marciniszyn (Meta) <mike.marciniszyn@gmail.com>
Link: https://patch.msgid.link/20260307105847.1438-3-mike.marciniszyn@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>