Bluetooth: hci_bcm: Fix clock (un)prepare

The hci_bcm driver currently does not prepare/unprepare the clock and
goes directly to enable, but as the documentation for clk_enable says,
clk_prepare must be called before clk_enable.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>