media: stkwebcam: move stk_camera_read_reg() scratch buffer to struct stk_camera

In stk_camera_read_reg() a single byte buffer is alloc-ed and
freed on every function call.  Since the size is known,
move the buffer to the struct stk_camera where it will be alloc-ed
and freed once.

Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>