powerpc/tm: Remove interrupt disable in __switch_to()

We currently turn IRQs off in __switch_to(0 but this is unnecessary as it's
already disabled in the caller.

This removes the IRQ disable but adds a check to make sure it is really off
in case this changes in future.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>