bootstd: Use blk uclass device numbers to set efi bootdev

When loading a file from a block device, efiload_read_file
was using the seq_num of the device (e.g "35" of virtio_blk#35)
instead of the block device id (e.g what you get from running
the corresponding device scan command, like "virtio 0")

This cause EFI booting from these devices to fail as an
invalid device number is passed to blk_get_device_part_str:

  Scanning bootdev 'virtio-blk#35.bootdev':
  distro_efi_read_bootflow_file start (efi,fname=<NULL>)
  distro_efi_read_bootflow_file start (efi,fname=<NULL>)
  setting bootdev virtio, 35, efi/boot/bootaa64.efi, 00000000beef9a40, 170800
  efi_dp_from_name calling blk_get_device_part_str
  dev=virtio devnr=35 path=efi/boot/bootaa64.efi
  blk_get_device_part_str (virtio,35)
  blk_get_device_by_str (virtio, 35)
  ** Bad device specification virtio 35 **
  Using default device tree: dtb/qemu-arm.dtb
  No device tree available
  0  efi          ready   virtio       1  virtio-blk#35.bootdev.par efi/boot/bootaa64.efi
  ** Booting bootflow 'virtio-blk#35.bootdev.part_1' with efi
  blk_get_device_part_str (virtio,0:1)
  blk_get_device_by_str (virtio, 0)
  No UEFI binary known at beef9a40 (image buf=00000000beef9a40,addr=0000000000000000)
  Boot failed (err=-22)

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Signed-off-by: Simon Glass <sjg@chromium.org>