net: smc91x: 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
  18709	    401	      0	  19110	   4aa6	drivers/net/ethernet/smsc/smc91x.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  18901	    201	      0	  19102	   4a9e	drivers/net/ethernet/smsc/smc91x.o

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