net: freescale: gianfar : constify dev_pm_ops structures.

dev_pm_ops are not supposed to change at runtime. All functions
working with dev_pm_ops provided by <linux/device.h> work with const
dev_pm_ops. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
  19057	    392	      0	  19449	   4bf9	drivers/net/ethernet/freescale/gianfar.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  19249	    192	      0	  19441	   4bf1	drivers/net/ethernet/freescale/gianfar.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>