[SERIAL] returning proper error from serial core driver

Fix the issue of returning 0 even in case of error from uart_set_info
function.  Now it returns the error EBUSY when it can not set new port.

Signed-off-by: Ram Gupta <r.gupta@astronautics.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>