[third_party/bcmdhd] Extract definitions of eventmsgs to crossdriver

fweh.cc in brcmfmac driver is refering this new struct, so extracting
the definitions to a new file in crossdriver.

Test: None, no functional change.
Bug: 52234
Change-Id: Ia221e8aba201f68d56d322a3144bdeb180184bb2
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/bcmdhd/+/428360
Reviewed-by: Karl Ward <karlward@google.com>
2 files changed