net: phy: initialize rc to zero to avoid returning garbage value

In the case where phydev->interrupts is not PHY_INTERRUPT_ENABLED
function vsc85xx_ack_interrupt is returning an uninitialized
garbage value.  Fix this by initializing rc to zero.

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