[nxpfmac] Download power files to FW.

When country code is set, attempt to download the relevant
power files to FW. Added support for ClearCountry()
as well.

Bug: 110627

Test: Manually tested setting and clearing country code on
the device. Added unit test to verify ClearCountry().

Change-Id: I72b342752f6b816b7d49c484f3147f330dd8b638
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/732322
Reviewed-by: Bjoern Johansson <bjoernj@google.com>
Commit-Queue: Karthik Krishnaswami <karthikrish@google.com>