irq: mips: replace cpumask_weight with cpumask_empty where appropriate

bcm6345_l1_of_init() 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>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>