net: tsec: make tsec_private a private structure

net: tsec: make tsec_private a private structure

Move the definition of tsec_private within the only file that makes use
of it.

This adds the benefit of include/tsec.h not referencing PKTBUFSRX (which
is set to CONFIG_SYS_RX_ETH_BUFFER, which we're trying to move to be
under CONFIG_NET dependency) anymore. Considering drivers/net/tsec.c is
only built if CONFIG_NET=y, this is fine.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>