x86/irq: simplify loop in unmap_domain_pirq

x86/irq: simplify loop in unmap_domain_pirq

The for loop in unmap_domain_pirq is unnecessary complicated, with
several places where the index is incremented, and also different
exit conditions spread between the loop body.

Simplify it by looping over each possible PIRQ using the for loop
syntax, and remove all possible in-loop exit points.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>