drm/i915/lpe_audio: Remove pointless irq_to_desc() usage

Nothing uses the result and nothing should ever use it in driver code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20201210194043.862572239@linutronix.de