net: lwm2m: fix build with bootstrap enabled
net: lwm2m: fix build with bootstrap enabled

After lwm2m async io was introduced with 32989a38f0,
one instance of function lwm2m_send_message() was left unchanged,
and makes build to fail when boostrap support is enabled.

Signed-off-by: Kiril Petrov <107338d8e53750e9172859f7bc39ad67ac7af543@gmail.com>

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