hwmon: (lm75) Fix write operations for negative temperatures

Writes into limit registers fail if the temperature written is negative.
The regmap write operation checks the value range, regmap_write accepts
an unsigned int as parameter, and the temperature value passed to
regmap_write is kept in a variable declared as long. Negative values
are converted large unsigned integers, which fails the range check.
Fix by type casting the temperature to u16 when calling regmap_write().

Cc: Iker Perez del Palomar Sustatxa <iker.perez@codethink.co.uk>
Fixes: e65365fe ("hwmon: (lm75) Convert to use regmap")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>