Input: serio: convert bus code to use drv_groups

The drv_attrs field of struct bus_type is going away soon, drv_groups
should be used instead.  This converts the serio bus code to use the
correct field.

Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>