net/phy: fix DP83865 phy interrupt handler

According to the DP83865 datasheet we need to clear
the interrupt status bit by writing a 1 to the
corresponding bit in INT_CLEAR (2:0 are reserved).

Proposed and tested by Thorsten.

Signed-off-by: Thorsten Schubert <tshu@msc-ge.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>