serial: 8250: Declare base port operations const

The base port operations dispatch table should never be modified
at runtime; the proper way to override base port operations is
to dup the port operations, modify the copy, and point port->ops
at the copy.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>