drivers: staging: speakup: serialio: only use platform specific SERIAL_PORT_DFNS.

drivers: staging: speakup: serialio: only use platform specific SERIAL_PORT_DFNS.

If SERIAL_PORT_DFNS isn't present by platform, it need be defined to
"nothing", like the 8250 serial driver does it.

All related macros also need be removed: IRQF_SHARED is defined in
"linux/interrupt.h", others will be defined when related architecture
has SERIAL_PORT_DFNS.

Or it will cause issue (for arc, with allmodconfig):

    CC [M]  drivers/staging/speakup/serialio.o
  drivers/staging/speakup/serialio.c:12:2: error: initializer element is not constant
    SERIAL_PORT_DFNS
    ^
  drivers/staging/speakup/serialio.c:12:2: error: (near initialization for 'rs_table[0].baud_base')
  drivers/staging/speakup/serialio.c:12:2: error: initializer element is not constant
  drivers/staging/speakup/serialio.c:12:2: error: (near initialization for 'rs_table[1].baud_base')
  drivers/staging/speakup/serialio.c:12:2: error: initializer element is not constant
  drivers/staging/speakup/serialio.c:12:2: error: (near initialization for 'rs_table[2].baud_base')
  drivers/staging/speakup/serialio.c:12:2: error: initializer element is not constant
  drivers/staging/speakup/serialio.c:12:2: error: (near initialization for 'rs_table[3].baud_base')

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>