stmmac: Fix another build warning.

drivers/net/stmmac/dwmac100_core.c: In function 'dwmac100_dump_mac_regs':
drivers/net/stmmac/dwmac100_core.c:47: warning: cast from pointer to integer of different size

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>