mgmt: mcumgr: grp: os_mgmt: Fix compilation warning

This is a follow up to commit 770482a4.

Add initialization of the `ok` variable to prevent the "may be
uninitialized" warning when `CONFIG_BOOTLOADER_MCUBOOT` is not
defined.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>