Bluetooth: Read flow control mode on AMP controller init

When initializing an AMP controller, read its current flow control
mode so that the correct value is used.

The AMP controller defaults to block based flow control and this
extra command is just to double check.

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