hns_enet: use cpumask_var_t for on-stack mask

On large SMP builds, we can run into a build warning:

drivers/net/ethernet/hisilicon/hns/hns_enet.c: In function 'hns_set_irq_affinity.isra.27':
drivers/net/ethernet/hisilicon/hns/hns_enet.c:1242:1: warning: the frame size of 1032 bytes is larger than 1024 bytes [-Wframe-larger-than=]

The solution here is to use cpumask_var_t, which can use dynamic
allocation when CONFIG_CPUMASK_OFFSTACK is enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>