Bluetooth: host: Use uintptr when checking for valid index

Align bt_le_per_adv_sync_get_index, bt_conn_index and
bt_le_ext_adv_get_index in use of uintptr_t.

This fixes an issue where the cast to uint8_t happened before the
assert for a valid index, which could lead to invalid pointers passing
this assert.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>