stmmac: Fix compilation error in mmc_core.c

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 <[email protected]>
Cc: Giuseppe Cavallaro <[email protected]>
Acked-by: Giuseppe Cavallaro <[email protected]>
Signed-off-by: David S. Miller <[email protected]>