media: s2255drv: remove redundant assignment to variable field

The variable 'field' is being assigned a value this is never read,
it is being updated in all the following if/else combinations. The
assignment is redundant and can be removed.

Addresses-Coverity: ("Unused value")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>