serial: mxs-auart: Fix mxs_auart_set_ldisc()

Commit 732a84a0 ("serial: core: Pass termios to set_ldisc() notifications")
changed the set_ldisc prototype.

At the time of this commit the mxs_auart driver did not implement set_ldisc,
so that's why it has not been converted.

Adapt also mxs_auart_set_ldisc() so that the following build warning can be
fixed:

drivers/tty/serial/mxs-auart.c:962:2: warning: initialization from
incompatible pointer type
  .set_ldisc      = mxs_auart_set_ldisc,
  ^
drivers/tty/serial/mxs-auart.c:962:2: warning: (near initialization
for 'mxs_auart_ops.set_ldisc')

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>