Bluetooth: hci_bcm: Use operation speed of 4Mbps only for ACPI devices

Not all Broadcom controller support the 4Mbps operational speed on UART
devices. This is because the UART clock setting changes might not be
supported.

  < HCI Command: Broadcom Write UART Clock Setting (0x3f|0x0045) plen 1
         01                                               .
  > HCI Event: Command Complete (0x0e) plen 4
       Broadcom Write UART Clock Setting (0x3f|0x0045) ncmd 1
         Status: Unknown HCI Command (0x01)

To support any operational speed higher than 3Mbps, support for this
command is required. With that respect it is better to not enforce any
operational speed by default. Only when its support is known, then allow
for higher operational speed.

This patch assigns the 4Mbps opertional speed only for devices
discovered through ACPI and leave all others at the default 115200.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>