hwmon: (i5k_amb) Replace BUG with WARN

hwmon: (i5k_amb) Replace BUG with WARN

Executing BUG as a result of an internal driver error seems to be a bit harsh.
Replace it with WARN and return -ENODEV if the condition is seen.

This also resolves the following compile warning seen with some random
configurations.

drivers/hwmon/i5k_amb.c: In function 'i5k_channel_pci_id':
drivers/hwmon/i5k_amb.c:492: warning: control reaches end of non-void function

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Darrick J. Wong <djwong@us.ibm.com>