Bluetooth: BR: improve br_sufficient_key_size

Currently, the minimum value of encryption key size is
`BT_HCI_ENCRYPTION_KEY_SIZE_MIN`.

Add a new Kconfig `BT_BR_MIN_ENC_KEY_SIZE`. It is used to set the
specific minimum encryption key size.
The default value is `BT_SMP_MIN_ENC_KEY_SIZE`. And it can be
configured if `BT_SMP_SC_ONLY` is not enabled.

Use `CONFIG_BT_BR_MIN_ENC_KEY_SIZE` as minimum encryption key size in
`br_sufficient_key_size`.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>