greybus: firmware: firmware image name has .tftf instead of .fw

That's how the bootrom-tool names it, and that's how the kernel should
expect it.

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