[media] av7110_hw: fix a sanity check in av7110_fw_cmd()

ARRAY_SIZE(buf) (8 elements) was intended instead of sizeof(buf) (16
bytes).  But this is just a sanity check and the callers always pass
valid values so this doesn't cause a problem.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>