tty: fix bluetooth scribbling on low latency flags

Bluetooth shouldn't be doing this as most drivers don't support the flag,
furthermore it shouldn't be needed with newer buffering. This becomes rather
more visible as the locking fixes make the abuse of low_latency visible as
spew on the users console/dmesg.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>