serial8250: Use dl_read()/dl_write() on Alchemy

Convert the 8250 Alchemy support code to make
use of the new dl_read()/dl_write() callbacks.

Signed-off-by: Magnus Damm <damm@opensource.se>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>