drivers: usb: nordic: Power consumption fix with cable detached

This commit fixes the issue with excess current being drawn
during sleep due to active HFCLK with external crystal.
Clock-related operations were moved to cable attachmend and
detachment handlers to ensure that HFCLK is not requested
when it's not needed.

Signed-off-by: Paweł Zadrożniak <pawel.zadrozniak@nordicsemi.no>