greybus: firmware: Fix incorrect firmware file's name

All the id-fields are 32 bit long instead of 16 bits and so we will need
8 characters per field instead of four. Also the stage field is only one
byte long and so needs just two characters to represent it.

Reported-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>