hwmon: (ina2xx) Fix word size register read and write operations

The driver uses be16_to_cpu and cpu_to_be16 to convert data in SMBus word
operations from chip to host byte order. However, the data passed from and to
the SMBus word API functions is in host byte order, not in chip byte order.
Conversion should therefore use swab16 instead of be16 to change the byte order.

Replace driver internal word conversion functions with SMBus API functions to
solve the problem.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org # 3.5+
Acked-by: Jean Delvare <khali@linux-fr.org>