hwmon: (ltc2978) Add polling for chips requiring it

Some of the LTC chips supported by this driver have to be polled
to ensure that they are ready to accept commands.

Signed-off-by: Michael Jones <mike@proclivis.com>
[Guenter Roeck: simplifications and formatting changes]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>