drivers: modem: ublox-sara-r4: implemented sendmsg
drivers: modem: ublox-sara-r4: implemented sendmsg

Implemented sendmsg

Tested using Sara U201 and the MQTT protocol library.

Signed-off-by: Hans Wilmers <8800578b51f022c8d8adb9606a8b3db4fedbdac6@wilmers.no>

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