staging: mt7621-mmc: Fix requesting of irq

The request of the irq line is not correctly handled by the current
code, platform_get_irq may fail and if the irq is requested via
devm_request_irq, its release is automatically done.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>