nfc: nci: uart: Constify struct tty_ldisc_ops

nfc: nci: uart: Constify struct tty_ldisc_ops

'struct tty_ldisc_ops' is not modified in this driver.

Constifying this structure 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
  11454	   3352	    256	  15062	   3ad6	net/nfc/nci/uart.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  11646	   3160	    256	  15062	   3ad6	net/nfc/nci/uart.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/c756755a72cdfde2877a18ddee01eaa4f633c220.1778443960.git.christophe.jaillet@wanadoo.fr
Signed-off-by: David Heidelberg <david@ixit.cz>