mariadb: use ${PN} for SYSTEMD_AUTO_ENABLE

Use ${PN} instead of hardcoding 'mariadb' so that everything works
fine in case of multilib.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>

meta-oe/recipes-support/mysql/mariadb_5.5.39.inc

@@ -31,7 +31,7 @@ GROUPADD_PARAM_${PN}-server = "--system mysql"
 
 SYSTEMD_PACKAGES = "${PN}-server"
 SYSTEMD_SERVICE_${PN}-server = "mysqld.service"
-SYSTEMD_AUTO_ENABLE_mariadb-server = "disable"
+SYSTEMD_AUTO_ENABLE_${PN}-server = "disable"
 
 EXTRA_OEMAKE = "'GEN_LEX_HASH=${STAGING_BINDIR_NATIVE}/gen_lex_hash'"