net: can: at91_can: constify attribute_group structures.

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

File size before:
   text	   data	    bss	    dec	    hex	filename
   6164	    304	      0	   6468	   1944	drivers/net/can/at91_can.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6228	    240	      0	   6468	   1944	drivers/net/can/at91_can.o

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