staging: ccree: use sizeof(*var) in kmalloc

Fix the following checkpatch warning:

CHECK: Prefer kmalloc(sizeof(*buff_mgr_handle)...)
over kmalloc(sizeof(struct buff_mgr_handle)...)

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>