samples: drivers: can: counter: use printf() instead of printk()

Use printf() instead of printk() for printing sample output. According to
the documentation Zephyr printk() and friends are for printing kernel debug
messages.

With printf() instead of printk() the CAN counter sample passes twister
test execution on native_posix and native_posix_64.

Fixes: #50570

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>