The xemac_l.h file that is written to the include directory under the PowerPC includes an incorrect define:
#define XEM_EMIR_TYPE_MASK 0x0000FF00UL /* Device type */
The documentation specifies that "TYPE" is in the lower byte. The correct mask value is 0x000000FFUL.
This problem has been fixed in the latest EDK 8.1i Service Pack, available at:
The first service pack containing the fix is EDK 8.1i Service Pack 2.