greybus: firmware: fix potential stack corruption

Use snprintf when generating the firmware name to avoid stack corruption
if the fixed-size buffer overflows.

Note that the current buffer size appears to expect 16-bit ids while
the they are actually 32-bit, something which could trigger the
corruption.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>