hwmon: (ltc2992) Fix less than zero comparisons with an unsigned integer

There are several occurrances of a less than zero error check on
a u32 unsigned integer. These will never be true. Fix this by making
reg_value a plain int.

Addresses-Coverity: ("Unsigned comparison against zero")
Fixes: e126370240e0 ("hwmon: (ltc2992) Add support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20201207142410.168987-1-colin.king@canonical.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>