Bluetooth: btusb: Add Broadcom patch RAM support

Bluetooth: btusb: Add Broadcom patch RAM support

After hardware reset, some BCM Bluetooth adapters obtain their initial firmware
from OTPROM chip. Once this initial firmware is running, the firmware can be
further upgraded over HCI interface with .hcd files provided by Broadcom. This
is also known as "patch RAM" support. This change implements that.

If the .hcd file is not found in /lib/firmware, BCM Bluetooth adapter continues
to operate with the initial firmware. Sample kernel log:
  hotplug: sys=firmware act=add fw=brcm/BCM20702A0-0a5c-22be.hcd dev=...
  Bluetooth: hci0: BCM: patch brcm/BCM20702A0-0a5c-22be.hcd not found

If the .hcd file is found, btusb driver pushes it to the BCM Bluetooth adapter and
it starts using the new firmware. Sample kernel log:
  hotplug: sys=firmware act=add fw=brcm/BCM20702A0-0a5c-22be.hcd dev=...
  Bluetooth: hci0: BCM: patching hci_ver=06 hci_rev=1000 lmp_ver=06 lmp_subver=220e
  Bluetooth: hci0: BCM: firmware hci_ver=06 hci_rev=1389 lmp_ver=06 lmp_subver=220e

Above, we can see that hci_rev goes from 1000 to 1389 as a result of the upgrade.

Signed-off-by: Petri Gynther <pgynther@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>