uart_get_baud_rate: stop mangling termios

Russell King noticed this one: We have to avoid replacing B0 when we pick a
baud rate for a "hung up" port.  Ugly but the proper fix is in the tty layer
and means changing the tty<->serial interfaces so we will defer that for now.

[akpm@linux-foundation.org: fix uninitialised var]
Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>