efi_loader: wrong type in wait_for_event

The UEFI spec defines parameter index of WaitForEvent as UINTN*.
So we should use size_t here.

I deliberately do not use UINTN because I hold a following patch
that will eliminate UINTN because uppercase types to not match
the U-Boot coding style.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Alexander Graf <agraf@suse.de>