ena: Initialize statistics before the interface is available

In [1], the FBSD community exposed a bug in the fbsd/ena driver.

Bug description:

Current function call order is as follows:

1. ena_attach()

1.1. ena_setup_ifnet()
1.1.1. Registration of ena_get_counter()
1.1.2. ether_ifattach(ifp, adapter->mac_addr);
1.2. Statistics allocation and initialization.

At point 1.1.2, when ether_ifattach() returns, the interface is available,
and stats can be read before they are allocated, leading to kernel panic.

Also fixed a potential memory leak by freeing the stats since they were
not freed in case the following calls failed.

Fix:

This commit moves the statistics allocation and initialization to happen
before ena_setup_ifnet()

[1] https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=268934

Fixes: 9b8d05b8ac78 ("Add support for Amazon Elastic Network Adapter (ENA) NIC")
Fixes: 30217e2dff10 ("Rework counting of hardware statistics in ENA driver")
MFC after: 2 weeks
Sponsored by: Amazon, Inc.