examples/rxtx_callbacks: fix HW timestamp config

examples/rxtx_callbacks: fix HW timestamp config

Since all PMDs doesn't implement per queue offload capabilities but
supports RX timestamping and also since rx_offload_capa includes all
rx_queue_offload_capa's. Hence moving the logic to enable HW timestamp
via DEV_RX_OFFLOAD_TIMESTAMP flag before device configuration so that
this application can work with all PMDs.

Fixes: cd1dadeb9b2a ("examples/rxtx_callbacks: support HW timestamp")

Signed-off-by: Harman Kalra <hkalra@marvell.com>
Reviewed-by: Jerin Jacob <jerinj@marvell.com>