[logging] Aling define guards on RegisterLogModule and LogAt.

Commit aligns define guards on `LogAt` macto and `RegisterLogModule` to
match. Previuosly `OT_SHOULD_LOG` was getting default value set as
platform defined and log level was buy default set to none which caused
misalignment.

src/core/common/log.hpp

@@ -77,7 +77,7 @@ constexpr uint8_t kMaxLogModuleNameLength = 14; ///< Maximum module name length
 
 constexpr uint16_t kMaxLogStringSize = OPENTHREAD_CONFIG_LOG_MAX_SIZE; ///< Max size of log string
 
-#if OT_SHOULD_LOG && (OPENTHREAD_CONFIG_LOG_LEVEL != OT_LOG_LEVEL_NONE)
+#if OT_SHOULD_LOG
 /**
  * Registers log module name.
  *