greybus: bootrom: Rename download_firmware() as find_firmware()

The download_firmware() function isn't downloading the firmware but just
finding if one is available or not. The same applies to the error
message printed by it.

Replace 'download' with 'find' to make it sound better.

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>