BTIF: Fix return types of interface getters

* They should return const pointer to interface structs
* Fixed return type for btif_rc_ctrl_get_interface, it should be
  "const btrc_ctrl_interface_t*" but was "btrc_interface_t*". Later
  casting masked this error, but it may hurt us in the future if not
  fixed

Test: make
Change-Id: I41b2366cce68b1338761d84cef9ab986fd3334df