[media] si2168: add one missing parenthesis

Fix following warnings:
si2168_cmd_execute() warn: add some parenthesis here?
si2168_cmd_execute() warn: maybe use && instead of &

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>