wifi: b43: Constify struct lpphy_tx_gain_table_entry

'struct lpphy_tx_gain_table_entry' 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
  16481	   6232	      0	  22713	   58b9	drivers/net/wireless/broadcom/b43/tables_lpphy.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  22305	    395	      0	  22700	   58ac	drivers/net/wireless/broadcom/b43/tables_lpphy.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-By: Michael Büsch <m@bues.ch>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/e33bc9e6dff4a5b6cd8d0ab5399aa1abac5bef9d.1722753127.git.christophe.jaillet@wanadoo.fr