[block] Use drive number as debug message stream ID

[block] Use drive number as debug message stream ID

We currently use the SAN device pointer as the debug message stream
identifier.  This pointer is not always available: for example, when
booting from a local disk there is no underlying SAN device.

Switch to using the drive number as the debug message colour stream
identifier, so that all block device debug messages may be colourised
consistently.

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