media: intel/ipu6: update the maximum supported csi2 port number to 6

IPU6EP on Meteor Lake SoC supports maximum 6 csi2 ports instead of 4.

Fixes: 25fedc02 ("media: intel/ipu6: add Intel IPU6 PCI device driver")
Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>