char: ipmi: constify bmc_dev_attr_group and bmc_device_type

File size before:
   text	   data	    bss	    dec	    hex	filename
  25678	   1024	     92	  26794	   68aa	drivers/char/ipmi/ipmi_msghandler.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  25806	    896	     92	  26794	   68aa	drivers/char/ipmi/ipmi_msghandler.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>