hwmon: (dme1737) Be less i2c-centric

The dme1737 driver support both LPC (ISA) and SMBus devices. At the
moment it's rather i2c-centric, and LPC variants use a fake i2c_client
for some operations.

In a near future, i2c_client will be allocated by i2c-core rather than
by the device drivers, so non-i2c drivers will not have one. As a
preparation step, change the driver code to no longer assume that
an i2c_client structure is always available. No functional change.
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Juerg Haefliger <juergh@gmail.com>