net/mlx4_core: fix handling return value of mlx4_slave_convert_port

The function can return negative values, so its result should
be assigned to signed variable.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/assign_signed_to_unsigned.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2046107

Fixes: fc48866f7 ('net/mlx4: Adapt code for N-Port VF')
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>