fs_enet/mii-fec.c: fix MII speed calculation

The MII speed calculation was based on the CPU clock (ppc_proc_freq),
but for MPC512x we must use the bus clock instead.

This patch makes it use the correct clock and makes sure we don't
clobber reserved bits in the MII_SPEED register.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: <netdev@vger.kernel.org>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>