trusty: cast-auth: fix incorrect return value on error path

The function returns a bool to identify the keytype, but would return an
error code on memory failure.  Change the function to avoid the memory
allocation failure path.

Bug: 332371854
Test: build.py qemu-generic-arm64-test-debug --test com.android.trusty.cast_auth.test
Change-Id: Idc0f4b2484ec5b00bf97eed3647196ef2da463bb