qfw: return failure when no kernel could be loaded

When we try to load a kernel via the QEMU firmware device, we currently
"return -1;" if no kernel was specified on the QEMU command line. This
leads to the usage output, which is confusing (since nothing on the
command line was really wrong), but also somewhat hides the actual error
message.

Return CMD_RET_FAILURE (1), as it's a proper error, and make the message
more clear that this is not only a "warning".

This helps to call this command in boot scripts, and to gracefully
continue if this doesn't work.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>