media: v4l2-ctrl: fix flags for DO_WHITE_BALANCE

Control DO_WHITE_BALANCE is a button, with read only and execute-on-write flags.
Adding this control in the proper list in the fill function.

After adding it here, we can see output of v4l2-ctl -L
do_white_balance 0x0098090d (button) : flags=write-only, execute-on-write

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>