serial: mxs-auart: fix gpio change detection on interrupt

mxs_auart_modem_status() did't detect gpio's state change
because s->mctrl_prev was modified before by mctrl_gpio_get().
The patch introduces mctrl_temp variable to fix the bug.

Signed-off-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>