net: tsec: Move rx_idx and tx_idx to struct tsec_private

At present rx_idx and tx_idx are declared as static variables
in the driver codes. To support multiple interfaces, move it to
struct tsec_private.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>