[media] lirc: Make struct file_operations pointer const

struct file_operations was made const in the drivers, but not in struct
lirc_driver:

drivers/staging/lirc/lirc_it87.c:365: warning: initialization discards qualifiers from pointer target type
drivers/staging/lirc/lirc_parallel.c:571: warning: initialization discards qualifiers from pointer target type
drivers/staging/lirc/lirc_serial.c:1073: warning: initialization discards qualifiers from pointer target type
drivers/staging/lirc/lirc_sir.c:482: warning: initialization discards qualifiers from pointer target type
drivers/staging/lirc/lirc_zilog.c:1284: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>