staging: greybus: light: fix attributes allocation

Fix allocation of attributes with the correct size, this also fix smatch
warning:

drivers/staging/greybus/light.c:293 channel_attr_groups_set()
warn: double check that we're allocating correct size: 8 vs 16

Signed-off-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>