V4L/DVB (7100): frontends/tda18271-common.c: fix off-by-one

This patch fixes an off-by-one error spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Reviewed-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>