i2c_posix: fix use of wrong device path

previously, get_devname() was used as the I2C device path, but on NuttX,
get_devname() is the device file which the driver creates. This patch
changes it, so the sematics are the same as on NuttX: both now use _bus
to decide to which I2C bus device to talk to.

I did not see any other use-cases than the led on ocpoc.

src/drivers/drv_led.h

@@ -57,8 +57,4 @@
 // set the queue size to the number of LED's, so that each led can be controlled individually
 static const int LED_UORB_QUEUE_LENGTH = BOARD_MAX_LEDS;
 
-#if defined(BOARD_RGBLED0_PATH)
-#define RGBLED0_DEVICE_PATH BOARD_RGBLED0_PATH
-#else
 #define RGBLED0_DEVICE_PATH "/dev/rgbled0"
-#endif