drivers: counter: Fix Smartbond counter get_rate

When timer calibration for SmartBond(tm) was added, opaque type
clock_control_subsystem_t that used to have device tree ordinal
number was changed to enum to allow values that are not in device
tree (like no clock selection).

During this process counter driver for SmartBond(tm) was not updated
accordingly, resulting in wrong frequency being reported to counter
driver.

Failure could be seen when samples/drivers/counter/alarm was execute on
da1469x_dk_pro board.

With this fix counter driver uses correct type when
clock_control_get_rate is called and counter test works again.

Signed-off-by: Jerzy Kasenberg <jerzy.kasenberg@codecoup.pl>