[POWERPC] Fix bugs in the hypervisor call stats code

There were a few issues with the HCALL_STATS code:

- PURR cpu feature checks were backwards
- We iterated one entry off the end of the hcall_stats array
- Remove dead update_hcall_stats() function prototype

I noticed one thing while debugging, and that is we call H_ENTER (to set
up the MMU hashtable in early init) before we have done the cpu fixups.
This means we will execute the PURR SPR reads even on a CPU that isnt
capable of it. I wonder if we can move the CPU feature fixups earlier.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>