hwmon: (ltc2945) Handle error case in ltc2945_value_store

ltc2945_val_to_reg errors were not being handled
which would have resulted in register being set to
0 (clamped) instead of being left alone.

Fixes: 6700ce03 ("hwmon: Driver for Linear Technologies LTC2945")

Signed-off-by: Jonathan Cormier <jcormier@criticallink.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>