hal: Fix incorrect wsa count update during platform_init

buf is not reset after each read_line_from_file call.
Hence buf is matching with wsa after first wsa thermal
zone is found. This results in wsa_count being more than 2.

Buffer is reset after each iteration to fix this issue.

Change-Id: I73b1bd350c0a8659e7a42b5ccdb7600935254ae6