[bt][intel] Improve readability of firmware loading flow

Following up on some comments in
https://fuchsia-review.googlesource.com/c/garnet/+/208126.

Test: Test soft and hard reboots on a Pixelbook and NUC. Repeat at
least 5 times.

Change-Id: I652a59b20b4e5eea8a143c550f239a4fbee262c0
8 files changed