hwmon: (it87) Add calls to smbus_enable/smbus_disable as required

Disable/re-enable access through SMBus for chip registers when they are
are being read or written.

For simple cases this is done at the same time as when a mutex is set,
however, within loops or during initialisation it is done separately.

Signed-off-by: Frank Crawford <frank@crawford.emu.id.au>
[groeck: Fixed multi-line alignment]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>