[third_party/bcmdhd] Extract ratespec logic.

Broadcom firmware specifies the current rate parameters (e.g. channel
bandwidth, MCS index) in the form of a "ratespec" (also called "rspec").
This change extracts ratespec parsing symbols and logic into the
crossdriver source set, for ease of use in brcmfmac.

Test: manually tested with a variety of ratespec values from a running
firmware on a live device. Unit tests will be added into the Fuchsia
tree in a following CL, after this CL makes the symbols available to

Bug: 52811
Change-Id: I1ee13a2020d214510c56934fc210a17a4b578a20
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/bcmdhd/+/483877
Reviewed-by: Kiet Tran <kiettran@google.com>
5 files changed