IB/hfi1: Constify struct flag_table

'struct flag_table' are not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
 302932	  40271	    112	 343315	  53d13	drivers/infiniband/hw/hfi1/chip.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
 311636	  31567	    112	 343315	  53d13	drivers/infiniband/hw/hfi1/chip.o

Link: https://lore.kernel.org/r/782b6a648bfbbf2bb83f81a73c0460b5bb7642a1.1720959310.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>