Blackfin: bfin_mac: set MDCDIV based on SCLK

Rather than hardcoding MDCDIV to 24 (which is correct for ~125mhz SCLK),
use the real algorithm so it gets set correctly regardless of SCLK.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Ben Warren <biggerbadderben@gmail.com>