parisc: iosapic: fix build breakage

drivers/parisc/iosapic.c:717: error: incompatible types in assignment

irq_desc::affinity was changed from cpumask_t to cpumask_var_t in
7f7ace0c

 (cpumask: update irq_desc to use cpumask_var_t)

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>