Staging: speakup: Fix getting port information

Commit f79b0d9c ("staging: speakup: Fixed warning <linux/serial.h>
instead of <asm/serial.h>") broke the port information in the speakup
driver: SERIAL_PORT_DFNS only gets defined if asm/serial.h is included,
and no other header includes asm/serial.h.

We here make sure serialio.c does get the arch-specific definition of
SERIAL_PORT_DFNS from asm/serial.h, if any.

Along the way, this makes sure that we do have information for the
requested serial port number (index)

Fixes: f79b0d9c ("staging: speakup: Fixed warning <linux/serial.h> instead of <asm/serial.h>")
Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: stable <stable@vger.kernel.org> # 3.18
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>