Documentation/target: Fix tcm_mod_builder.py build breakage

Fix build breakage and set the protocol identifier based on the parameter.

Fixes: 9ac8928e ("target: simplify the target template registration API")
Fixes: e4aae5af ("target: change core_tpg_register prototype")

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>