mgmt/osdp: Flush RX buffer before sending data

Partial packets in the RX buffers cause the subsequent packet to be
treated as malformed. The RX buffer can have partial data if the sender
is too slow in sending the packet of if there is an interruption in
transmission mid-way.

To avoid any issues due to such partials, flush the uart channel before
sending the command/response.

Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>