docbook: fix 8250 naming in device-drivers

8250 driver has been (re)renamed to 8250_core.c by commit
9196d8acd7f91758872108958dfded7684628444. Follow that change to fix the
following error when building htmldocs:

  docproc: /work/cross/linux//drivers/tty/serial/8250/8250.c: No such file or directory

Signed-off-by: Vincent Stehlé <v-stehle@ti.com>
Cc: Rob Landley <rob@landley.net>
Cc: linux-doc@vger.kernel.org