V4L/DVB (4861): Remove the need of a STD array for drivers using video_ioctl2

video_ioctl2 will auto-generate standard entries at ENUM_FMT.
Also, now, a driver may return a subset of the video array at
the return, to be stored as the current_norm.
For example, a driver may ask for V4L2_STD_PAL. At return,
driver may change it to V4L2_STD_PAL_B. This way, a futher call
to G_STD will return the exact detected video std.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>