efi_loader: fix a parameter check at CreateEvent()

efi_loader: fix a parameter check at CreateEvent()

The commit 21b3edfc9644 ("efi_loader: check parameters of CreateEvent")
enforces a strict parameter check at CreateEvent(). On the other hand,
UEFI specification version 2.7, section 7.1, says:

    The EVT_NOTIFY_WAIT and EVT_NOTIFY_SIGNAL flags are exclusive. If
    neither flag is specified, the caller does not require any notification
    concerning the event and the NotifyTpl, NotifyFunction, and
    NotifyContext parameters are ignored.

So the check should be mitigated so as to comply with the specification.
Without this patch, EDK2's Shell.efi won't be started.

Fixes: 21b3edfc9644 ("efi_loader: check parameters of CreateEvent")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>