net/mlx4: Fix firmware command timeout during interrupt test

Currently interrupt test that is part of ethtool selftest runs the
check over all interrupt vectors of the device.
In mlx4_en package part of interrupt vectors are uninitialized since
mlx4_ib doesn't exist. This causes NOP FW command to time out.
Change logic to test current port interrupt vectors only.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>