wifi: Add support for multiple radio stats

Previously, we've always assumed that we have only one instance of
radio stats fetched in link layer stats. This is no longer true for the
newer devices which support multiple radios.

Also,
Changed the timestamp member from uint32_t to uint64_t.

Bug: 36148086
Test: The error logs no longer seen on newer devices.
Change-Id: I048a1db7cc0bfb0dc0dacafff2156f42a8ae1e63