[media] v4l2-ctrls: fix rounding calculation

Commit 958c7c7e ("[media] v4l2-ctrls: fix corner case in round-to-range code") broke
controls that use a negative range.

The cause was a s32/u32 mixup: ctrl->step is unsigned while all others are signed. So
the result type of the expression '(ctrl)->maximum - ((ctrl)->step / 2)' became unsigned,
making 'val >= (ctrl)->maximum - ((ctrl)->step / 2)' true, since '((u32)-128) > 128'
(if val = -128, maximum = 128 and step = 1).

So carefully cast (step / 2) to s32.

There was one cast of step to s32 where it should have been u32 because both offset and
step are unsigned, so casting to signed makes no sense there. You do need a cast to u32
there, because otherwise architectures that have no 64-bit division start complaining
(step is a u64).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>