net: aquantia: Fix aq_vec_isr_legacy() return value

The irqreturn_t type is an enum or an unsigned int in GCC.  That
creates to problems because it can't detect if the
self->aq_hw_ops->hw_irq_read() call fails and at the end the function
always returns IRQ_HANDLED.

drivers/net/ethernet/aquantia/atlantic/aq_vec.c:316 aq_vec_isr_legacy() warn: unsigned 'err' is never less than zero.
drivers/net/ethernet/aquantia/atlantic/aq_vec.c:329 aq_vec_isr_legacy() warn: always true condition '(err >= 0) => (0-u32max >= 0)'

Fixes: 970a2e98 ("net: ethernet: aquantia: Vector operations")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>