tifm: convert bus code to use dev_groups

The dev_attrs field of struct bus_type is going away soon, dev_groups
should be used instead.  This converts the tifm bus code to use the
correct field.

Cc: Alex Dubov <oakad@yahoo.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>