stmmac: remove the STBus bridge setting from the GMAC code (v3)

This patch removes a piece of code (actually commented)
only useful for some ST platforms in the past.

This kind of setting now can be done by using the platform
callbacks provided in linux/stmmac.h (see the stmmac.txt for
further details).

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