serial: of: CONFIG_SERIAL_8250 is always set

The only other user of this code was the nwp-serial driver, but that
is now gone, so we can remove a couple of #ifdef statments in this
driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>