[PATCH] generic_serial: fix decoding of baud rate

Commit d720bc4b

 partially removed a
private implementation of baud speed decoding.  However it doesn't seem
to be complete: after the speed is decoded, it is still being used as an
index to a local speed table (array overrun, no doubt).

This was found by Graham Murray who noticed it caused a 2.6.19 regression
with the SX driver: https://bugs.gentoo.org/170554

Signed-off-by: Daniel Drake <dsd@gentoo.org>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>