dmaengine: mxs: remove NO_IRQ check

The mxs_chan->chan_irq variable is guaranteed to never be NO_IRQ,
as it gets assigned the result of platform_get_irq() that returns
either a valid positive interrupt number, or a negative failure
code that leads to the channel not being used.

This removes the redundant check, eliminating one more instance
of NO_IRQ.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>