qed: initialize return rc to avoid returning garbage

in the case where qed_slowpath_irq_req is not called, rc is not
assigned and so qed_int_igu_enable will return a garbage value.
Fix this by initializing rc to 0.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Yuval Mintz <Yuval.Mintz@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>