tpm/tpm_crb: fix priv->cmd_size initialisation

priv->cmd_size is never initialised if the cmd and rsp buffers reside
at different addresses.  Initialise it in the exit path of the function
when rsp buffer has also been successfully allocated.

Fixes: aa77ea0e ("tpm/tpm_crb: cache cmd_size register value.").
Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>