[media] dvb: si21xx: buffer overflow in si21_writeregs()

"len" is user controlled and can be up to 255.  Anything more than 59
will cause a buffer overflow so we need to add a test for that.

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