Bluetooth: Use kzalloc instead of kmalloc/memset

Replace kmalloc+memset by kzalloc
for better readability and simplicity.

This addresses the cocci warning below:

WARNING: kzalloc should be used for d, instead of kmalloc/memset

Signed-off-by: Kang Minchul <tegongkang@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>