stmmac: fix PWRDWN into the PMT register for global unicast.

MAC devices use the RWKPKTEN and MGKPKTEN bits of the PMT Control/Status
register to generate power management events.
So this patch is to properly set the RWKPKTEN [BIT(2)] inside the
PMT register (needed in case of global unicast).

Reported-by: Aditi SHARMA <aditi-hed.sharma@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre TORGUE <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>