serial: move the flags into the tty_port field

Fortunately the serial layer was designed to use the same flag values but
with different names. It has its own SUSPENDED flag which is a free slot in
the ASYNC flags so we allocate it in the ASYNC flags instead.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>