Bluetooth: controller: Fix advertising random delay resolution calc

In commit d5836195 ("Bluetooth: controller: Increase advertising
random delay resolution"), the resolution of random_delay was
increased from 8-bit to 16-bit.  Due to this switch the result
of HAL_TICKER_US_TO_TICKS() can now be a 0, which causes the following
crash:
***** Kernel OOPS! *****
Current thread ID = 0x200043f0
Faulting instruction address = 0x17914
Fatal fault in ISR! Spinning...

Let's make sure we don't pass a 0 to ticker_update() by increasing
the result of HAL_TICKER_US_TO_TICKS() by 1.

Signed-off-by: Michael Scott <mike@foundries.io>