[media] stb0899: don't go past DiSEqC msg buffer

As reported by spatch:
	drivers/media/dvb-frontends/stb0899_drv.c:720 stb0899_send_diseqc_msg() error: buffer overflow 'cmd->msg' 6 <= 7

The buffer size is 6 and not 8. Anyway, the best is to use sizeof(),
to avoid such mistakes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>