Docu: genericirq.rst: fix irq-example

A code example was missing the pointer to dereference a variable.

Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230824110109.18844-1-pstanner@redhat.com