media: dvb_frontend: fix return error code

The correct error code when a function is not defined is
-ENOTSUPP. It was typoed wrong as -EOPNOTSUPP, with,
unfortunately, exists, but it is not used by the DVB core.

Thanks-to: Geert Uytterhoeven <geert@linux-m68k.org>
Thanks-to: Arnd Bergmann <arnd@arndb.de>

To make me revisit this code.

Fixes: a9cb97c3 ("media: dvb_frontend: be sure to init dvb_frontend_handle_ioctl() return code")
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>