ipsec offload: never return error from the newkey/spdadd callbacks

Returning an error causes premature termination of if_foreach_sleep()
loop over the interfaces. Whatever problem we have with the specific
interface trying to install an element, should not prevent an attempt to
install the same element into all other interfaces.

Noted by: Ariel Ehrenberg
Sponsored by: NVidia networking
MFC after: 1 week