sensors: default_rtio_sensor: fix build warning

Fix a build warning on 64 bit architectures:

zephyr/drivers/sensor/default_rtio_sensor.c:238:17: error: format '%zu'
expects argument of type 'size_t', but argument 2 has type 'uint32_t'
{aka 'unsigned int'}

num_channels type changed to uint32_t in 96175fcc.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>