samples: Bluetooth: hci_rpmsg: Fix event buffer handling
samples: Bluetooth: hci_rpmsg: Fix event buffer handling

This fixes an issue where we dropped the send buffer when
ipc_service_send() timed out waiting for a shared memory buffer.

We now wait in the IPC RX endpoint, but this shouldn't be a problem for
other IPC users, as PR #43806 introduces separate workqueues
per instance, with configurable priorities.

This also includes small changes regarding the IPC service return value.

Signed-off-by: Jonathan Rico <fa71e224653c2b3d230eecbd20dc417397790bdf@nordicsemi.no>

galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr