Bluetooth: controller: openisa: Fix radio ISR signature

Non-direct ISRs are supposed to take a void pointer as an argument,
unliike direct ones, which take no arguments. Since the radio ISR is not
declared as direct, the void pointer argument was missing, likely due to
a copy-paste mistake from the nordic LLL, which indeed uses a direct ISR
for the radio ISR.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>