Bluetooth: Host: Use `memset` to initialize `psa_mac_operation_t`

In `db_hash_setup()` the state object for MAC operations was initialized
using `psa_mac_operation_init()`. This function was not always optimized
or inlined.

A way to reduce stack usage is to use `memset()` and set the object to
0. This is one of the option documented to initialize
`psa_mac_operation_t` object.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>