efi_loader: compliance Simple Text Input Ex Protocol

We cannot expect the buffers passed to the input protocols to be zero
filled. If only modifier keys are pressed, we have to return EFI_NOT_READY
but we still have to fill the key structure.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>