net: fec: fix build error at m68k platform

reproduce:
  wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
  chmod +x ~/bin/make.cross
  git checkout 4d494cdc


  make.cross ARCH=m68k  m5272c3_defconfig
  make.cross ARCH=m68k

drivers/net/ethernet/freescale/fec.h:262:0: warning: "FEC_R_DES_START" redefined
 #define FEC_R_DES_START(X) ((X == 1) ? FEC_R_DES_START_1 : \
 ^
drivers/net/ethernet/freescale/fec.h:158:0: note: this is the location of the previous definition
 #define FEC_R_DES_START  0x3d0 /* Receive descriptor ring */
 ^
drivers/net/ethernet/freescale/fec.h:265:0: warning: "FEC_X_DES_START" redefined
 #define FEC_X_DES_START(X) ((X == 1) ? FEC_X_DES_START_1 : \

...

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>