tpm_crb: Clean-up and refactor check for idle support

Refactor TPM idle check to tpm_crb_has_idle(), and reduce paraentheses
usage in start method checks

[jarkko: Fine-tuned the commit message.]

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Stuart Yoder <stuart.yoder@arm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>