common/sfc_efx/base: refine adding encap action to set

common/sfc_efx/base: refine adding encap action to set

1) Invalid encap. header ID is always set by default.
   Do not set it again when adding the action.

2) Encap. header ID validity check is missing in the
   action set allocation helper. Introduce it.

Fixes: 3907defa5bf0 ("common/sfc_efx/base: support adding encap action to a set")
Cc: stable@dpdk.org
Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>