rv: Return init error when registering monitors

Monitors generated with dot2k have their registration function (the one
called during monitor initialisation) return always 0, even if the
registration failed on RV side.
This can hide potential errors.

Return the value returned by the RV register function.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Tomas Glozar <tglozar@redhat.com>
Cc: Juri Lelli <jlelli@redhat.com>
Cc: Clark Williams <williams@redhat.com>
Cc: John Kacur <jkacur@redhat.com>
Link: https://lore.kernel.org/20250723161240.194860-6-gmonaco@redhat.com


Reviewed-by: Nam Cao <namcao@linutronix.de>
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>