AP_HAL_Linux: allow to change spi device state

Allowing to change the SPI device state allows us to save the
information whether the device was already initialized and avoid 2
separate drivers to initialize it.

libraries/AP_HAL_Linux/SPIDriver.h

@@ -28,6 +28,8 @@ public:
     uint8_t transfer (uint8_t data);
     void transfer (const uint8_t *data, uint16_t len);
     void set_bus_speed(enum bus_speed speed);
+    void set_state(State state) override { _state = state; }
+    State get_state() override { return _state; }
 
 private:
     uint16_t _bus;
@@ -36,6 +38,7 @@ private:
     AP_HAL::DigitalSource *_cs;
     uint8_t _mode;
     uint8_t _bitsPerWord;
+    State _state = State::UNKNOWN;
     uint32_t _lowspeed;
     uint32_t _highspeed;
     uint32_t _speed;