ure: improve checksum offloading

This patch fixes three issues:
(1) Initially, set the hwassist flags correctly when enabling

transmit checksum offload for TCP/IPv6 and UDP/IPv6.

(2) Keep the hwassist flags in sync with the capabilities when

changing txcsum.

(3) Keep the hwasssit flags in sync with the capabilities when

changing txcsum6.

Without this patch, transmit checksum offloading for TCP/IPv6 and
UDP/IPv6 is never used and transmit checksum offloading for IPv4,
TCP/IPv4 and UDP/IPv4 is always used, even if disabled via
ifconfig ue? -txcsum.

Reviewed by: Timo Völker
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D54974