AP_HAL_Linux: fix use of 0-length array

0-length arrays are supported in C but forbidden in C++. GCC allows it
but clang is more strict:

../../libraries/AP_HAL_Linux/SPIDriver.cpp:75:35: fatal error: no matching constructor for initialization of 'Linux::SPIDeviceDriver [0]'
SPIDeviceDriver SPIDeviceManager::_device[0];
                                  ^
../../libraries/AP_HAL_Linux/SPIDriver.h:20:7: note: candidate constructor (the implicit move constructor) not viable: requires 1 argument, but 0 were provided
class SPIDeviceDriver : public AP_HAL::SPIDeviceDriver {
      ^
../../libraries/AP_HAL_Linux/SPIDriver.h:20:7: note: candidate constructor (the implicit copy constructor) not viable: requires 1 argument, but 0 were provided
../../libraries/AP_HAL_Linux/SPIDriver.h:25:5: note: candidate constructor not viable: requires 9 arguments, but 0 were provided
    SPIDeviceDriver(const char *name, uint16_t bus, uint16_t subdev, enum AP_HAL::SPIDeviceType type, uint8_t mode, uint8_t bitsPerWord, int16_t cs_pin, uint32_t lowspeed, uint32_t highspeed);
    ^
1 error generated.

libraries/AP_HAL_Linux/SPIDriver.cpp

@@ -72,10 +72,14 @@ SPIDeviceDriver SPIDeviceManager::_device[] = {
 };
 #else
 // empty device table
-SPIDeviceDriver SPIDeviceManager::_device[0];
+SPIDeviceDriver SPIDeviceManager::_device[] = { };
+#define LINUX_SPI_DEVICE_NUM_DEVICES 0
 #endif
 
+#ifndef LINUX_SPI_DEVICE_NUM_DEVICES
 #define LINUX_SPI_DEVICE_NUM_DEVICES ARRAY_SIZE(SPIDeviceManager::_device)
+#endif
+
 const uint8_t SPIDeviceManager::_n_device_desc = LINUX_SPI_DEVICE_NUM_DEVICES;
 
 SPIDeviceDriver::SPIDeviceDriver(const char *name, uint16_t bus, uint16_t subdev, enum AP_HAL::SPIDeviceType type, uint8_t mode, uint8_t bitsPerWord, int16_t cs_pin, uint32_t lowspeed, uint32_t highspeed):