cpufreq: Fix up conversion to hotplug state machine

The function cpufreq_register_driver() returns zero on success and since
commit 27622b06 ("cpufreq: Convert to hotplug state machine")
erroneously a positive number. Due to the "if (x) assume_error" construct
all callers assumed an error and as a consequence the cpu freq kworker
crashes with a NULL pointer dereference.

Reset the return value back to zero in the success case.

Fixes: 27622b06

 ("cpufreq: Convert to hotplug state machine")
Reported-by: Borislav Petkov <bp@alien8.de>
Reported-and-tested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: peterz@infradead.org
Cc: rjw@rjwysocki.net
Link: http://lkml.kernel.org/r/20160920145628.lp2bmq72ip3oiash@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>