serial/8250_hp300: Missing 8250 register interface conversion bits

commit 2655a2c7 ("8250: use the 8250
register interface not the legacy one") forgot to fully switch one
instance of struct uart_port to struct uart_8250_port, causing the
following compile failure:

drivers/tty/serial/8250/8250_hp300.c: In function ‘hpdca_init_one’:
drivers/tty/serial/8250/8250_hp300.c:174: error: ‘uart’ undeclared (first use in this function)
drivers/tty/serial/8250/8250_hp300.c:174: error: (Each undeclared identifier is reported only once
drivers/tty/serial/8250/8250_hp300.c:174: error: for each function it appears in.)

This went unnoticed in -next, as CONFIG_HPDCA is not set to y by
allmodconfig.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Philip Blundell <philb@gnu.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>