net/sfc: fix error path inconsistency

net/sfc: fix error path inconsistency

At the fail label, there's a statement to set general errno and
error message. However, before the label is reached, a custom
error message can be set by the code which parses actions.
This custom (action-specific) message, when present,
must not be replaced by the general one.

Fixes: 662286ae61d2 ("net/sfc: add actions parsing stub to MAE backend")
Cc: stable@dpdk.org
Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>