Check if an IRQ is mapped before releasing it.

This will simplify future EEH code by allowing unconditional unmapping
of IRQs.
Acked-by: NCyril Bur <cyrilbur@gmail.com>
Signed-off-by: NDaniel Axtens <dja@axtens.net>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>