We presently deadlock in low-latency mode because the receive code holds
port.lock while calling into the tty code to perform echoing.  The tty code
calls back into the driver, which then takes port.lock.

Fix that by dropping the lock around the echo call.
Acked-by: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>