cmd: bootefi: Don't show usage help if EFI binary fails.

If an EFI binary returns an error code EFI_INVALID_PARAMETER, we show the
usage help for the bootefi command:

    Shell> exit 0x8000000000000002
    ## Application failed, r = 2
    bootefi - Boots an EFI payload from memory

    Usage:
    bootefi <image address>[:<image size>] [<fdt address>]
      - boot EFI payload
    bootefi bootmgr [fdt address]
      - load and boot EFI payload based on BootOrder/BootXXXX variables.

        If specified, the device tree located at <fdt address> gets
        exposed as EFI configuration table.

This makes no sense.

Fixes: 296faf4f ("cmd: bootefi: re-organize do_bootefi()")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>