media: cx25840: g_std operation really implements querystd operation

cx25840 driver g_std operation queries the currently detected video signal,
however this is what querystd operation should do, so let's rename the
handler.

None of the existing cx25840 driver users ever called the g_std operation,
one of them calls querystd on each of its subdevs but then the result is
only used to implement VIDIOC_QUERYSTD (as it should).

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>