sfxge(4): fix warnings from VS2015 C compiler (C4057)

Fix two level 4 warnings
"C4057: 'function': 'const uint8_t *' differs in indirection to
slightly different base types from 'caddr_t'"; 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/D18124