V4L2: Add manual white balance control.

Adds support for V4L2_CID_RED_BALANCE and
V4L2_CID_BLUE_BALANCE. Only has an effect if
V4L2_CID_AUTO_N_PRESET_WHITE_BALANCE has
V4L2_WHITE_BALANCE_MANUAL selected.

Signed-off-by: Dave Stevenson <dsteve@broadcom.com>
config: Add CONFIG_VETH for virtual ethernet devices