[media] technisat-usb2: fix typo in variable name

The variable txlen was used instead of rxlen in boundary check.
(copy-paste error)

Signed-off-by: Felipe Pena <felipensp@gmail.com>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>