serial: st-asc: Fix unused variable warning for non DT.

This patch fixes a below warning reported by kbuild test robot.

drivers/tty/serial/st-asc.c:727:28: warning: 'asc_match' defined but
 not used [-Wunused-variable]

The code used in DT case is now ifdefed under CONFIG_OF to fix this
warning.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Reported-by: Kbuild Test Robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>