Blackfin EMAC driver: Fix Ethernet communication bug (dupliated and lost packets)

Fix Ethernet communication bug(dupliated and lost packets)
in RMII PHY mode- dont call mac_disable and mac_enable during
10/100 REFCLK changes - mac_enable screws up the DMA descriptor chain

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>