i.MX: Fix FEC code for MDIO operation selection

According to the FEC chapter of i.MX25 reference manual

When writing the MMFR register, bit 29 and 28 select the requested operation.
 * 10 means read operation with valid MII mgmt frame
 * 11 means read operation with non compliant MII mgmt frame
 * 01 means write operation with valid MII mgmt frame
 * 00 means write operation with non compliant MII mgmt frame

So while bit 28 does change beween read/write for valid MII mgmt frame, the
mening is inverted for non compliant MII mgmt frame.

Bit 29 on the other hand means read/write whatever the type of mgmt frame
involved.

So this patch change the operation selection from bit 28 to bit 29 as it is
more generic.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Signed-off-by: Jason Wang <jasowang@redhat.com>
