[media] staging: atomisp: lm3554: fix sparse warnings(was not declared. Should it be static?)

Fix "symbol 'xxxxxxx' was not declared. Should it be static?" sparse warnings.

Signed-off-by: Chen Guanqiao <chen.chenchacha@foxmail.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>