sfxge(4): use simpler code to check hash algorithm type

[arybchik]

Summary

The API which is used to list supported hash flags verifies
hash algorithm choice before writing the output. This check
is based on a switch() statement which has only two options
and no distinctive actions to be conducted for each of them.
Use simpler code instead of switch() to improve readability.

Submitted by: Ivan Malov <ivan.malov at oktetlabs.ru>
Sponsored by: Solarflare Communications, Inc.

Test Plan

amd64 tinderbox kernel builds tested
driver loaded/unloaded, interfaces up/down, traffic works fine