tpm_crb: fix crb_req_canceled behavior

The req_canceled() callback is used by tpm_transmit() periodically to
check whether the request has been canceled while it is receiving a
response from the TPM.

The TPM_CRB_CTRL_CANCEL register was cleared already in the crb_cancel
callback, which has two consequences:

* Cancel might not happen.
* req_canceled() always returns zero.

A better place to clear the register is when starting to send a new
command. The behavior of TPM_CRB_CTRL_CANCEL is described in the
section 5.5.3.6 of the PTP specification.

CC: stable@vger.kernel.org
Fixes: 30fc8d13 ("tpm: TPM 2.0 CRB Interface")
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>