devlink: fix encap mode manupulation

devlink: fix encap mode manupulation

DEVLINK_ATTR_ESWITCH_ENCAP_MODE netlink attribute carries enum. But the
code assumes bool value. Fix this by treating the encap mode in the same
way as other eswitch mode attributes, switching from "enable"/"disable"
to "basic"/"none", according to the enum. Maintain the backward
compatibility to allow user to pass "enable"/"disable" too. Also to be
in-sync with the rest of the "mode" commands, rename to "encap-mode".
Adjust the help and man page accordingly.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>