samples/mgmt/mcumgr/smp_svr: Switch to FIXED_PARTITION_ macros
samples/mgmt/mcumgr/smp_svr: Switch to FIXED_PARTITION_ macros

The commit switches code from using FLASH_AREA_* macros to
FIXED_PARTITION_* macros.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

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