ifinfo: improve output of hwassist value

In addition to print the hexadecimal number hwassist, also print
the symbolic names of the corresponding CSUM_* flags.

Reviewed by: tuexen
Differential Revision: https://reviews.freebsd.org/D55055
Event: Wiesbaden Hackathon 2026
(cherry picked from commit d84870d90bafe75eb87759df0ab41ff0214c866e)