efi_loader: disk: Fix CONFIG_BLK breakage

When using CONFIG_BLK, there were 2 issues:

  1) The name we generate the device with has to match the
     name we set in efi_set_bootdev()

  2) The device we pass into our block functions was wrong,
     we should not rediscover it but just use the already known
     pointer.

This patch fixes both issues.

Signed-off-by: Alexander Graf <agraf@suse.de>