hwmon: (lm90) Fix warnings

hwmon: (lm90) Fix warnings

With some configuration option combinations, we get the following
warnings:

drivers/hwmon/lm90.c: In function 'lm90_detect':
drivers/hwmon/lm90.c:1114: warning: 'chip_id' may be used uninitialized
in this function
drivers/hwmon/lm90.c:1114: warning: 'reg_config1' may be used
uninitialized in this function
drivers/hwmon/lm90.c:1114: warning: 'reg_convrate' may be used
uninitialized in this function
drivers/hwmon/lm90.c:1187: warning: 'reg_emerg2' may be used
uninitialized in this function
drivers/hwmon/lm90.c:1187: warning: 'reg_status2' may be used
uninitialized in this function

We can solve these easily by reading the register values first and
checking for errors later. These errors should be very rare, even in
the case of failed detection, so this change has no impact on
performance. And this makes checkpatch.pl happier.

Reported-by: Guenter Roeck <guenter.roeck@ericsson.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>