the device

you may encounter problems with :
- hfp/hsp , a2dp profile
- activate/disactivate bluetooth with bluedevil plasmoid.

to solve these problems you must supply the firmware of the device according system loads this firmware instead of a default one when pc starts .

in a user console execute the following command to download the firmware file( Note : it seems that plugable updates regularly the driver)
in /lib/firmware/brcm
Copy the firmware file to the /lib/firmware/brcm folder and rename the file to BCM20702A1-0a5c-21e8.hcd:
     sudo mv fw-0a5c_21e8.hcd /lib/firmware/brcm/BCM20702A1-0a5c-21e8.hcd
why renaming ? because system with kernel 4.12.14 expects this name

reboot pc

check in boot.msg there is no more message about failing to load firmware BCM20702A1-0a5c-21e8 :
bluetooth hci0: Direct firmware load for brcm/BCM20702A1-0a5c-21e8.hcd failed with error -2

inspired from

interesting doc about problem with some devices (bose , plantronics etc.) and new bluetooth features

other interesting doc