efi_loader: run CreateEvent() notify function based on flags

The UEFI specification states that the tpl, function and context
arguments are to be ignored if neither EVT_NOTIFY_WAIT or
EVT_NOTIFY_SIGNAL are specified.  This matches observed behaviour with
an AMI EDK2 based UEFI implementation.

Skip calling the notify function if neither flag is present.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Acked-By: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>