cpmac: fix compilation errors against undeclared BUS_ID_SIZE

With the removal of BUS_ID_SIZE, cpmac was not fully
converted to use MII_BUS_ID_SIZE as it ought to. This
patch fixes the following cpmac build failure:
 CC      drivers/net/cpmac.o
drivers/net/cpmac.c: In function 'cpmac_start_xmit':
drivers/net/cpmac.c:563: warning: comparison of distinct pointer types lacks a cast
drivers/net/cpmac.c: In function 'cpmac_probe':
drivers/net/cpmac.c:1112: error: 'BUS_ID_SIZE' undeclared (first use in this function)
drivers/net/cpmac.c:1112: error: (Each undeclared identifier is reported only once
drivers/net/cpmac.c:1112: error: for each function it appears in.)

Reported-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>