greybus: loopback: Fix warning on 32-bit build

gb_loopback_ro_avg_attr() is using "/" to divide two 64-bit integer,
causing a reference to __aeabi_uldivmod() that is not availalbe on 32-bit.
Instead, use do_div().

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>