TTY: hvc: convert to use DRIVER_ATTR_RW

We are trying to get rid of DRIVER_ATTR(), and the hvc driver's
attribute can be trivially changed to use DRIVER_ATTR_RW().

Cc: Jiri Slaby <jslaby@suse.com>
Cc: <linuxppc-dev@lists.ozlabs.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>