hwmon: (lm90) Simplify handling of extended local temp register

The optional extended local temperature register can never have
address 0, as this address is already used by another register. Thus
we can get rid of flag LM90_HAVE_LOCAL_EXT and simply rely on
reg_local_ext being non-zero to determine if a given chip has this
extension or not. This makes the code more simple.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Stijn Devriendt <sdevrien@cisco.com>
Cc: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>