us ticker read - should not return timestamp_t type?

As the title says, us_ticker_read() should probably return timestamp_t return value.

```
/** Read the current counter
 *
 * @return The current timer's counter value in microseconds
 */
uint32_t us_ticker_read(void);

/** Set interrupt for specified timestamp
 *
 * @param timestamp The time in microseconds to be set
 */
void us_ticker_set_interrupt(timestamp_t timestamp);
```
