8250: use container_of() instead of casting

The 8250 driver structure uart_8250_port took advantage of the fact
that the struct uart_port was the first member of its structure and
used an explicit cast to convert to the derived class. Replace the
explicit casts with container_of() for safety and clarity.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>