net: stmmac: fixed enh_desc set always zero

This patch fixed that enh_desc value is always zero.
Due to calling order of stmmac_selec_desc_mode(), enh_desc value is always zero.
Even though mac is set to use enhanced dma descriptor, if enh_desc is zero,
functions related dma descriptor are not working correctly.

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>