ure: improve receive checksum offloading

Let the receive checksum offload for TCP/IPv6 and UDP/IPv6 be
controlled by ifconfig rxcsum6 and not by ifconfig rxcsum.
While there, make the code more compact and improve stlye.9
conformity.

Reviewed by: Timo Völker
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55188