altera_uart: Fixup type usage of port flags

port->flags is of type upf_t, which corresponds to UPF_* flags.

ASYNC_BOOT_AUTOCONF is an unsigned integer, which happen to
be the same as UPF_BOOT_AUTOCONF.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Acked-by: Tobias Klauser <tklauser@distanz.ch>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>