hwmon: (lm90) Add missing assignment

Coverity reports the following error.

>>>     CID 1364474:  Error handling issues  (CHECKED_RETURN)
>>>     Calling "lm90_read_reg" without checking return value (as is done
>>>     elsewhere 28 out of 29 times).
532             lm90_read_reg(client, LM90_REG_R_REMOTE_LOWH);
533             if (val < 0)
534                     return val;

Fixes: 10bfef47 ("hwmon: (lm90) Read limit registers only once")
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>