clang: remove build warnings

* -Wunused-but-set-variable warning in module-lcd.c (variable 'cnt' set but not used)

* -Wnon-literal-null-conversion warning in module-led.c (expression which evaluates to zero treated as a null pointer constant of type 'time_t * _Nullable' (aka 'long *'))

* -Wunused-but-set-variable warning in oscam-conf-chk.c (variable 'nfilts' set but not used)

* -Wunused-but-set-variable warning in oscam-config.c (variable 'line' set but not used)

* -Wpointer-sign warning in oscam-net.c (passing 'uint32_t *' (aka 'unsigned int *') to parameter of type 'socklen_t * _Nonnull' (aka 'int *') converts between pointers to integer types with different sign)