drivers: clock_control_nrf: Prevent break from becoming dead code

When both NRF_CLOCK_HAS_XO_TUNE and NRF_CLOCK_HAS_PLL evaluate to 0,
one break statement can end up not associated with any case and become
dead code. Refactor a bit the related switch to avoid such situation.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>