net/mlx5: Fix build break when CONFIG_SMP=n

Avoid using the kernel's irq_descriptor and return IRQ vector affinity
directly from the driver.

This fixes the following build break when CONFIG_SMP=n

include/linux/mlx5/driver.h: In function ‘mlx5_get_vector_affinity_hint’:
include/linux/mlx5/driver.h:1299:13: error:
        ‘struct irq_desc’ has no member named ‘affinity_hint’

Fixes: 6082d9c9 ("net/mlx5: Fix mlx5_get_vector_affinity function")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
CC: Randy Dunlap <rdunlap@infradead.org>
CC: Guenter Roeck <linux@roeck-us.net>
CC: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Israel Rukshin <israelr@mellanox.com>
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>