serial_txx9: Use assigned device numbers

The serial_txx9 driver have abused device numbers (major 4, minor 128) if
CONFIG_SERIAL_TXX9_STDSERIAL was not set.  This patch makes the driver use
proper device numbers assigned for it (major 204, minor 196-203).  I
suppose a typical user of this driver set CONFIG_SERIAL_TXX9_STDSERIAL to Y
(i.e.  use "ttyS0"), so this patch would not cause big compatibility issue.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>