stmmac: Fix compilation error in mmc_core.c

Fix this error:

  CC      drivers/net/ethernet/stmicro/stmmac/mmc_core.o
drivers/net/ethernet/stmicro/stmmac/mmc_core.c: In function 'dwmac_mmc_ctrl':
drivers/net/ethernet/stmicro/stmmac/mmc_core.c:143:2: error: implicit
  declaration of function 'pr_debug' [-Werror=implicit-function-declaration]

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>