TTY: amiserial, provide carrier helpers

This is a preparation for a switch to tty_port_block_til_ready. We
need amiga_carrier_raised and amiga_dtr_rts. The implementation is
taken from startup, shutdown and current block_til_ready.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>