vp9_spatial_svc_encoder: fix -bit-depth arg parsing
vp9_spatial_svc_encoder: fix -bit-depth arg parsing

use arg_parse_enum_or_int like vpxenc. this also fixes a warning as
arg_parse_enum is not currently declared in args.h.

Change-Id: If9ce258d6adb6286eb86f529083929d5fe2b3a56

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