media: tm6000: Fix unused value in vidioc_try_fmt_vid_cap()

Coverity warns of an unused value:

assigned_value: Assign the value of the variable f->fmt.pix.field
to field here,  but that stored value is overwritten.
before it can be used.
919	field = f->fmt.pix.field;
920

value_overwrite: Overwriting previous write to field with
the value of V4L2_FIELD_INTERLACED.
921	field = V4L2_FIELD_INTERLACED;

Fixes: ed57256f ("[media] tm6000: fix G/TRY_FMT")
Signed-off-by: Zeng Jingxiang <linuszeng@tencent.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>