genirq/affinity: replace cpumask_weight with cpumask_empty where appropriate

__irq_build_affinity_masks() calls cpumask_weight() to check if
any bit of a given cpumask is set. We can do it more efficiently with
cpumask_empty() because cpumask_empty() stops traversing the cpumask as
soon as it finds first set bit, while cpumask_weight() counts all bits
unconditionally.

Signed-off-by: Yury Norov <yury.norov@gmail.com>