V4L/DVB (9448): Bug: fix array size

Bug: a string which contains 4 digits needs an array
of size 5. The fifth character will hold the terminating '\0'

Signed-off-by: Reinhard Nissl <rnissl@gmx.de>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>