Bluetooth: Improve Three-wire UART configuration handling

The configuration request/response messages contain a configuration
field which contains the sliding window size (amount of unacked reliable
packets that can be pending). This patch makes sure that we configure
the correct size (minimum of local and remote values) and use it when
determining whether to send new packets or not.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>