[media] v4l: omap4iss: csi2: Replace manual if statement with a subclk field

Instead of manually checking whether the CSI2 module is CSI2a or CSI2b
in order to select the right subclock to enable/disable, add a subclk
field to the iss_csi2 structure, initialize it with the corresponding
subclock value and use it at runtime.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>