serial: mxs-auart: Remove irq from private structure

The irq number is only used inside the probe function, so there is no need
to keep it in the private mxs_auart_port structure.

Use a local 'irq' variable for storing the irq number instead.

Also make its type of 'int' as platform_get_irq() may fail and return a
negative number.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>