[media] pvrusb2: don't go past buf array

That fixes the following smatch warning:
	drivers/media/usb/pvrusb2/pvrusb2-hdw.c:4909 pvr2_hdw_state_log_state() error: buffer overflow 'buf' 256 <= 4294967294

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