serial: 16550: Make serial_io/out_shift available to debug mode

The ns16550 driver needs serial_in_shift() and serial_out_shift()
when compiled in debug UART mode, so shift the DM_SERIAL check a
little to make these functions available.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>