tty/serial_core: Introduce lock mechanism for RS485

tty/serial_core: Introduce lock mechanism for RS485

Introduce an homogeneous lock system between setting and using the rs485
data of the uart_port.

This patch should not be split into multiple ones in order to avoid
leaving the tree in an unstable state.

Acked-by: Nicolas Ferre <[email protected]>
Reviewed-by: Alan Cox <[email protected]>
Suggested-by: Alan Cox <[email protected]>
Cc: Nicolas Ferre <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: One Thousand Gnomes <[email protected]>
Signed-off-by: Ricardo Ribalda Delgado <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>