cpu_control: Remove voltage reset

I think the voltage reset is silly. The voltage will be changed for us
when the cpu scales (which occurs upon governor change)
cpu_control: Fix up error handling for user input, prevent overflow.
cpu_control: Cleanup init/exit error handling
cpu_control: Remove dead code

Some of the dead code tries to add another opp for the mpu (aka cpu).
This doesn't really work because if someone else has obtained the
existing frequency table with cpufreq_frequency_get_table(), then we
can't update their pointer. We would also need to update
omap2plus-cpufreq's freq_table pointer.

The whole thing is actually doable though, because nobody calls
cpufreq_frequency_get_table() besides the governors, which we could
restart to cause them to perform that call again.