sfxge(4): fix warnings from VS2015 C compiler (C4244)

Fix level 4 warning
"C4244: '+=': conversion from 'unsigned int' to 'uint16_t', possible loss
of data"; 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/D18120