drivers: bluetooth: siwx917: Fix issue with controller to host flow control

The SiWx917 BLE controller currently does not support
HCI Command 0x0C35 i.e. Host Number of Completed Packets Command
Disabling the ACL Flow Control avoids sending this command during
SMP Pairing process and therefore avoiding an ASSERT

Signed-off-by: Ramana Gudipudi <ragudipu@silabs.com>