ixp4xx-microcode: use snprintf in IxNpeMicrocode.h

ixp4xx-microcode: use snprintf in IxNpeMicrocode.h

Replace sprintf() calls with snprintf() to bound writes into the
fixed-size filename[] and slnk[] stack buffers. While the current
inputs are hash-pinned firmware images, snprintf provides defense
in depth against buffer overflows if the format string output ever
exceeds buffer capacity.

Signed-off-by: OrbisAI Security <mediratta01.pally@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23172
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>