MFC 330882: Simplify error handling in t4_tom.ko module loading.

• Change t4_ddp_mod_load() to return void instead of always returning success. This avoids having to pretend to have proper support for unloading when only part of t4_tom_mod_load() has run.
• If t4_register_uld() fails, don't invoke t4_tom_mod_unload() directly. The module handling code in the kernel invokes MOD_UNLOAD on a module whose MOD_LOAD fails with an error already.