Fix I2C init error on BeagleBone Black

This init method verifies that a device exists at the given address.

It was writing a zero byte value to the bus.  This triggered a
bug in the Linux kernel I2C bus OMAP2 driver for the BeagleBone.
The driver returns an invalid write error when msg->len is 0:
https://github.com/beagleboard/linux/blob/4.14/drivers/i2c/busses/i2c-omap.c#L665

The solution is to write the value 'x' instead of ''.

Refer to Adafruit_Blinka PR #42 for more information:
adafruit/Adafruit_Blinka#42 (comment)

Author: pdp7

adafruit_bus_device/i2c_device.py

@@ -57,11 +57,11 @@ class I2CDevice:
                 device.write(bytes_read)
     """
     def __init__(self, i2c, device_address):
-        # Verify that a device with that address exists.
+        # write non-zero byte value to verify device exists at address
         while not i2c.try_lock():
             pass
         try:
-            i2c.writeto(device_address, b'')
+            i2c.writeto(device_address, b'x')
         except OSError:
             raise ValueError("No I2C device at address: %x" % device_address)
         finally: