stmmac: remove the mmc code (v3)

DWMAC Management Counters (MMC) are not fully support.
The minimal support added in the past allowed to
only disable counters (if present) and mask their
interrupts.
This patch prepares the driver to support the MMC
removing obsolete code.

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