[efi] Use efi_handle_name() instead of efi_handle_devpath_text()

Signed-off-by: Michael Brown <mcb30@ipxe.org>
[efi] Use efi_handle_name() instead of efi_devpath_text() where applicable

Using efi_devpath_text() is marginally more efficient if we already
have the device path protocol available, but the mild increase in
efficiency is not worth compromising the clarity of the pattern:

  DBGC ( device, "THING %p %s ...", device, efi_handle_name ( device ) );

Signed-off-by: Michael Brown <mcb30@ipxe.org>