powerpc/ibmebus: Get rid of the IRQ mapping in ibmebus_free_irq()

ibmebus_free_irq() frees the IRQ but does not remove its mapping, which
results in stale entries in the map.

This fixes it by adding a call to irq_dispose_mapping() in
ibmebus_free_irq().

Signed-off-by: Sebastien Dugue <sebastien.dugue@bull.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>