condition: Check that subsystem is enabled in ConditionSecurity=tpm2

condition: Check that subsystem is enabled in ConditionSecurity=tpm2

Instead of succeeding when either the firmware reports a TPM device
or we find a TPM device, let's check that the firmware reports a TPM
device and the TPM subsystem is enabled in the kernel.

To check whether the subsystem enabled, we check if the relevant
subdirectory in /sys exists at all.