bsr: convert bsr_class to use dev_groups

The dev_attrs field of struct class is going away soon, dev_groups
should be used instead.  This converts the bsr bsr_class code to use
the correct field.

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