net: aquantia: fixed buffer overflow

The overflow is detected by smatch:

drivers/net/ethernet/aquantia/atlantic/aq_pci_func.c: 175
  aq_pci_func_free_irqs() error: buffer overflow 'self->aq_vec' 8 <= 31

In reality msix_entry_mask always restricts number of iterations.
Adding extra condition to make logic clear and smatch happy.

Signed-off-by: Nikita Danilov <nikita.danilov@aquantia.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>