power: smb135x-charger: fix the type of dc_psy_type

The driver uses enum to track whether dc_psy_type is valid. It sets
it to -EINVAL if its not valid. However, enums are unsigned integers
and checking it against negative values is always going to fail. Fix
this by changing the type to an integer.

Change-Id: Icb8a23a1b4660503f68edf6cdcca7d39c1ebfcb2
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>