mfd: ezx-pcap: Don't pass pcap pointer as subdev drvdata.

Abusing subdev drvdata is not needed anymore, as all pcap subdevs are now
retrieving the pcap pointer from their parent device.

This change removes a leftover coming from early versions of ezx-pcap and its
subdevs drivers.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>