[media] stb6100: fix buffer length check in stb6100_write_reg_range()

We are checking sizeof() the wrong variable!

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>