[media] helene: fix a warning when printing sizeof()

drivers/media/dvb-frontends/helene.c: In function 'helene_write_regs':
drivers/media/dvb-frontends/helene.c:312:5: warning: format '%lu' expects argument of type 'long unsigned int', but argument 5 has type 'unsigned int' [-Wformat=]
     "wr reg=%04x: len=%d vs %lu is too big!\n",
     ^

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