cxgb4: Constify struct thermal_zone_device_ops

'struct thermal_zone_device_ops' are not modified in this driver.

Constifying these structures moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   2912	   1064	      0	   3976	    f88	drivers/net/ethernet/chelsio/cxgb4/cxgb4_thermal.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   3040	    936	      0	   3976	    f88	drivers/net/ethernet/chelsio/cxgb4/cxgb4_thermal.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/e6416e0d15ea27a55fe1fb4e349928ac7bae1b95.1748164843.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Jakub Kicinski <kuba@kernel.org>