media: v4l: fwnode: Only zero the struct if bus type is set to V4L2_MBUS_UNKNOWN

In order to prepare for allowing drivers to set the defaults for a given
bus, make zeroing the struct conditional based on detecting the bus.
All callers now set the bus type to zero which allows only zeroing the
remaining bus union.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Tested-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>