HID: ft260: fix an error message in ft260_i2c_write_read()

The "len" variable is uninitialize.

Fixes: 6a82582d ("HID: ft260: add usb hid to i2c host bridge driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>