drivers: can: loopback: rename receive function
drivers: can: loopback: rename receive function

Rename the CAN loopback driver receive function to make the code easier to
understand. Add proper check against NULL for RX callback function
pointers.

Signed-off-by: Henrik Brix Andersen <24a3f13c22635bab658e39fc5f1e63c575328aae@vestas.com>

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