powerpc: Update comments in irqflags.h

The comment on TRACE_ENABLE_INTS is incorrect, and appears to have
always been incorrect since the code was merged. It probably came from
an original out-of-tree patch.

Replace it with something that's correct. Also propagate the message to
RECONCILE_IRQ_STATE(), because it's potentially subtle.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>