samples: subsys: mgmt: mcumgr: Fix FS part of smp_svr example

After introducing of Virtual File System Switch (#6318)
it i required to mount file system in order to use it.
This patch add to the example code which mounts NFFS
file system in the example.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>