greybus: loopback: drop unneeded casts for void pointers

There is no need to cast a void pointer to a particular type.
Drop the casts used in this way, mainly in the attribute definition
macros.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>