irda: Convert function pointer arrays and uses to const

Making things const is a good thing.

(x86-64 defconfig with all irda)
$ size net/irda/built-in.o*
   text	   data	    bss	    dec	    hex	filename
 109276	   1868	    244	 111388	  1b31c	net/irda/built-in.o.new
 108828	   2316	    244	 111388	  1b31c	net/irda/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>