[media] lnbh25: Fix lnbh25_attach() function return type

If CONFIG_DVB_LNBH25 is disabled, a stub static inline function is
defined that just prints a warning about the driver being disabled
but the function return type was wrong which caused a build error.

Fixes: e025273b

 ("[media] lnbh25: LNBH25 SEC controller driver")

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>