powerpc: Fix bugs introduced by sysfs changes

Rusty's patch to change our sysfs access to various registers
to use smp_call_function_single() introduced a whole bunch of
warnings. This fixes them. This version also fixes an actual
bug in here where it did mtspr instead of mfspr when reading
the files

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>