sfxge: fix pointer parameter/value signedness mismatch warnings

TLV routines use 'uint8_t *', NVRAM code uses caddr_t. Just cast to
required type to fix the warning.

Required to build with -Werror=pointer-signg.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4391