wifi: wlcore: testmode: Constify strutc nla_policy

'struct nla_policy' is not modified in this driver.

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

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   5062	    528	      0	   5590	   15d6	drivers/net/wireless/ti/wlcore/testmode.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   5178	    404	      0	   5582	   15ce	drivers/net/wireless/ti/wlcore/testmode.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/78810e3ebb74ddbd3a4538f182bf1143b89baba7.1731332414.git.christophe.jaillet@wanadoo.fr