MFC r340887

sfxge(4): fix warnings from VS2015 C compiler (C4245)

Fix level 4 warning
"C4245: 'initializing': conversion from 'int' to 'uint32_t',
signed/unsigned mismatch" warning; no functional changes.

Submitted by: Andrew Lee <alee at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18121