AP_HAL: AP_HAL_Linux: perfect I2CDevice::transfer()

According to man 3 ioctl, ioctl returns other values than -1 on success.
So loop while ioctl returns -1.

Furthermore, there is no necessity to initialise r with -EINVAL,

Signed-off-by: Ralf Ramsauer <ralf.ramsauer@othr.de>

libraries/AP_HAL_Linux/I2CDevice.cpp

@@ -177,13 +177,13 @@ bool I2CDevice::transfer(const uint8_t *send, uint32_t send_len,
     i2c_data.msgs = msgs;
     i2c_data.nmsgs = nmsgs;
 
-    int r = -EINVAL;
+    int r;
     unsigned retries = _retries;
     do {
         r = ::ioctl(_bus.fd, I2C_RDWR, &i2c_data);
-    } while (r < 0 && retries-- > 0);
+    } while (r == -1 && retries-- > 0);
 
-    return r >= 0;
+    return r != -1;
 }
 
 bool I2CDevice::read_registers_multiple(uint8_t first_reg, uint8_t *recv,