serial: diminish usage of struct serial_uart_config

This structure might have made sense many years ago, but at this
point it is only used in one specific driver, and referenced in
stale comments elsewhere.  Rather than change the sunsu.c driver,
simply move the struct to be within the exclusive domain of that
driver, so it won't get inadvertently picked up and used by other
serial drivers going forward.  The comments referencing the now
driver specific struct are updated accordingly.

Note that 8250.c has a struct that is similar in usage, with the
name serial8250_config; but is 100% independent and untouched here.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>