sfxge(4): fix warnings from VS2015 C compiler (C4310)

Fix level 4 warning
"C4310: cast truncates constant value";
no functional changes.

Submitted by: Andrew Lee <alee at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18119