tty: serial: imx: remove the use of MXC_INTERNAL_IRQS

As the part of the effort to enable SPARE_IRQ for imx platform,
the macro MXC_INTERNAL_IRQS will be removed.  The imx serial driver
has a references to it for a decision on flags of request_irq call
based on rtsirq is beyond MXC_INTERNAL_IRQS.  However the searching
on imx platform code tells that rtsirq will never be beyond
MXC_INTERNAL_IRQS.  That said, the check, consequently the reference
to MXC_INTERNAL_IRQS are not needed, so remove them.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Cc: linux-serial@vger.kernel.org
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>