mgmt/mcumgr/lib: Correct zephyr_grp log module name

The zephyr_grp log module name was incorrectly named
MGMT_SETTINGS and module registration has been using module name
mgmt_zephyr_basic, which is also incorrect.
Both have been changed to mcumgr_zephyr_grp.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>