greybus: bootrom: Don't print duplicate error messages

On failing to find a firmware image, we get these today:

bootrom 1-3.3.1: Firmware request for ara_00000126_00001001_fffe0001_ffe5001a_s2l.tftf has failed : -12
bootrom 1-3.3.1: gb_bootrom_firmware_size_request: failed to download firmware (-12)

Which are more or less duplicate, as they print error for the same root
cause.

With this patch this is all we get now:

bootrom 1-3.3.1: failed to download ara_00000126_00001001_fffe0001_ffe5001a_s2l.tftf firmware (-12)

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>