cmd: bootefi: error handling bootefi selftest

If bootefi selftest is executed and a problem with the device-tree
installation occurs, efi_install_fdt() writes sensible error messages.
It never returns EFI_INVALID_PARAMETER. It neither makes sense to check
for EFI_INVALID_PARAMETER nor to show the usage help for the bootefi
command in this case.

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>