efi_loader: correct efi_disk_register

efi_disk_register should return as status code (efi_status_t).

Disks with zero blocks should be reported as 'not ready' without throwing
an error.

This patch solves a problem running OpenBSD on system configured with
CONFIG_BLK=n (e.g. i.MX6).

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