parisc: Re-enable interrupts early

Since kernel 3.9 we re-enable interrupts quite late due to commit c207a76b
("parisc: only re-enable interrupts if we need to schedule or deliver signals
when returning to userspace"). At that time the parisc kernel had no dedicated
IRQ stack, and this commit prevented kernel stack overflows.

But since commit 200c8804

 ("parisc: implement irq stacks") we now have an
IRQ stack, so we may be safe now.  And when CONFIG_DEBUG_STACKOVERFLOW=y is
enabled, we can even check at runtime for overflows.

Signed-off-by: Helge Deller <deller@gmx.de>