MMC: sdio_uart, remove unused member from sdio_uart_port

tty from struct sdio_uart_port is unused. Proper refcounted tty in
tty_port->tty is used instead. So remove the member from that
structure.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>