stmmac: remove STMMAC_DEBUG_FS

the STMMAC_DEBUG_FS Koption is now removed from the
driver configuration and this support will be built
by default when DEBUG_FS is present. This can also be
useful on building driver verification.

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