net: stmmac: fixed operator typo

This patch fixed operator typo from & to ==.
Due to incorrect operator, the result is incorrect.

Signed-off-by: Byungho An <bh74.an@samsung.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>