[media] v4l2-clk: add new definition: V4L2_CLK_NAME_SIZE

Make all v4l2-clk's clock name use V4L2_CLK_NAME_SIZE definition.

In future, if the string is increased we just need to change the
V4L2_CLK_NAME_SIZE once.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>