chcr_ktls: use tls_offload_context_tx and driver_state like other drivers

chcr_ktls uses the space reserved in driver_state by
tls_set_device_offload, but makes up into own wrapper around
tls_offload_context_tx instead of accessing driver_state via the
__tls_driver_ctx helper.

In this driver, driver_state is only used to store a pointer to a
larger context struct allocated by the driver.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>